,.\ .. 

., 

DEPARTMENT OF THE INTERIOR 

UN"rTED ~STATES ·GEOLOGICAL SURVEY 

GEOH.GE OTIS SMITH, DIRECTOR 

PHOFESSIONAL PAPER 85-A 

1.,JIE Ol=tiG-IN 01~ COLl~~lANITE DEPOSITS 

BY 

HOY'"!", S. GALE 

:Pnhlished August 11, 191:~ 

ADVANCE CHAPTER FROM PROFESSIONAL PAPER 85 

"CONTRIBUTIONS TO GENERAL GE_OLOGY, 1913n 

'V ASHINGTON 

GOVl~H.Nl\'lENT PRINTING O~..,FIQE 

1913 
PREFACE. 

The present paper is the fiTst of a new series to be published by the Geological SurYey 

under the title.'' Contributions to general geology." Of late years SurYey authors have become 

contributors to scientific a~1d technical journals to an extent that suggests the need of an official 

channel for papers of a certain type. Contribution to these outside jo.urnals is a valuable phase 

of the Survey's ·activity and will continue; but this method. of publication has certain linlitq,-

tions by reasmi of both the capacity and the circulation of these journals. The ti1ne has come 

for beginning a volmne that will afford opportunity for the publication of short papers and 

preliminary reports of a character not well adapted to publication in any of the forms heretofore 

used by the Survey. 

It is significant that so 1nany of the Survey geologists are making scientific contributions 

of general ipterest thn,t represent results incidental to other inYestigations or that are of the 

nature of by-products of work whose imn1ediate purpose is econ6n1ic. This is becnuse the 

so-called econmnic work of the Survey is really scientific. It is scientific in n1ethod and scientific in results.' It represents the application of science to the solution of problen1s whose 

1nai.n ben,ring n1ay be utilitarian but which, if successfully solYed, will afford by-products in 

results and principles that will yield greater ulti1nate possibilities of use to n1an. In order to 

encourage greater breadth of observation and investigation among the geologists· and to pro- . 

n1ote the scientific possibilities of their professional work, n1eans should be provided for pro~pt 

publication of such papers in a penuanent forn1 that will c~mmend itself to both author and 

reader. l?rovision has been n1ade since 1902 for the current publication of short papers relating 

specifically to econon1ic geology, and the new series here established is intended to 1nake similar 

provision for scientific papers relating to gener'al geology. 

In advance of the printing of the full volume, separates, each including one or n1ore papers; 

will be issued to the nun1ber of 10 or 12 a year as the manuscript and illustrations are ready, 

without waiting for n1aterial for the full volmne to be in hand or even pronlised. These separates 

will be paged continuously, so that the Yolume will be simply a consolidation of the separates, 

without change of pagination, a distinct advantage for bibliographic reference. The date of 

publication will be printed on the title-page of each separate. 

'I'he papers included in these" Contributions to general geology" Dlay relate to any phase of 

geology, proYided it possesses general interest-petrology, paleontology, stratigraphy, glaciology, 

structural geology, etc. This Yolume is intended not as a catch-all for current odds .. and ends, 

but as a dignified collection of scientific contributions, each worthy in!mportance of subject, 

value of results, and quality of treatn1ent for separate publication as a bulletin or professional 

paper if it were of sufficient length. · 

1 

I 

.r 

·~ 

CON'rl1IB.U'riONS 110 -(;·ENElli\.L GEOLOGY, 1913. 

THE OHJGIN OF COLEMANITE DEPOSITS. 

By IIOYT S. GALE. 

INTRODUCTION. 

A brief s~udy of several of the n1ore in1portant borate deposits in southern California 

during 1912 has suggested to the writer son1e ideas believed to be n~~v regarding the origin 

of colemnnite. The theory presented here has not yet been entirely proved, but as there is 

.much in its favor and as it affords suggestions and a working basis for further observation, 

which the writer also hopes to have opportunity to 1nake, it seems proper to place ~he n1atter 

briefly on record. 

This paper is based chie.fl.y on a short personal study of the borate deposits ill Ventura 

County, Cftl., supplmnentecl by 1nore cursor.y exan1inations of slinilar deposits in the vic.inity 

of Death Valley, and includes a .review of published data on this subject and of personal 

com1ntmications fron1 those who have visited deposits elsewhere. A report discussing the 

econmnic geology of the colemanite deposits of Ventura County and the mining operations 

on them will appear in another place. 1 It is believed that these deposits afford exceptional 

opportunities for the study of the relationships of the colemanite ores. 

VOLCANIC ORIGIN OF BORIC ACID. 

It is generally recognized that boric acid il1 considerable quantities is an original constituent in the waters and gases given off with volcanic emanations. In fact,· the Tuscan 

fu:maroles have been an i1nportant conunercial source of boric acid in Italy for a long tin1e, 

and in the past, possibly even to the present tin1e, ahnost all the. boric acid brought into the 

European market was derived from this source. There is abundant evidence of the presence 

of boric acid in volcanic emanations in m.any parts of the world. On the other hand, boron 

is so· rare a constituent of rock-forming n1inerals that it fonns an almost inappreciably small 

percentage of the earth's rock mass as a whole. It is therefore natural in studying the genesis 

of boron n1inerals or deposits to look primarily for evidence of a possible origin in volcanic 

mnanations. Such an origil1 is believed to be in fact strongly indicated by the relations to be 

observed in n1ost borate deposits that have b~en studied. 

HYPOTHESES REGARDING ORIGIN OF COLEMANITE. 

The imn1ediate origin of the colmnanite ores of the southern part of the. Grea~ Basin has 

been discussed by several writers and geologists. W. H. Storn1s was perhaps the first to 

provide the explanation which has been more or less fully accepted since. In describing th~ 

1 d{)posits at Calico, near Daggett, Cal., he says: 2 · 

'.l'o me it seems that what is now one of the most valuable deposits of mineral in the State was at one time the site. 

of a. Tert;:iary ln.ke of considerable but as yet undetermined size; that although subjected to the same oscillation as the 

remainder of the region, a basin formed, in which the waters collected, c~ying with them the mineral salts derived 

1 Gale, Hoyt S., 'l'he borate deposits of Ventura County, Cal.: U.S. Geol. Survey Bull. 540-0,1913. 

2 Storms, W. H., Minoml resources of San Bernardino County, Cal.: California State Min. Bur. Eleventh Ann. Rept., p. 346, 1893. 

1896-13-2 3 
4 CONT.RIBUTIONS TO GENERAL GBOLOGY, 1913. 

hom the rocks of tho neighboring cou~1try; that finally the climatic conditions became such that the supply of w~LI;er 

was less than tho loss by absorption and evaporation, and the waters of the lake slowly diminished, it finally disappearing entirely, leaving on the floor of the lake a thick deposit of calcium borate of snowy whiteness. 

After the deposition of the borax bed a general subsidence of the region occurred, ·the waters of the great Tertiary 

lake once more coveri11g the whole country. Again the sands and finer sedimentary material_:_the erosion of the 

mountains-were canied clown and found a resti11g place on the floor of the lake, the borax beds being finally covered. 

Subsequent deforn1ation of these stratahas tilted and erosion exposed the borate beds. 

Can1pbell 1 adopted a similar explanation of the genesis of the deposits at the same locality: 

The ho1·ax of De<:tth Valley, as well as that near Daggett, occurs in a regular stratum, interbedded with the sein:iincluratcd sands and clays that make up the bulk of the strata. These beds are generally regarded as of Tertiary age, 

and they are supposed to have been deposited in inclosed bodies of water. 

* * * * * * * * * 

The principal deposit of boron salts [near Daggett] occurs at Borate, about 12 miles north of Dagge.tt, in the vicinity 

oi' the old Calico mining district. The mineral found here is borate of lime, or colemanite, and it occurs as a bedded 

deposit from 5 to 30 feet i11 thickness, interstratified in lake s<?diments. 

Spurr, 2 in discussing the character and formation of the Tertiary lake beds, says, with 

special reference to the borate of lin1e deposits: "The internal evidence of these beds sho\vs that 

n1uch of the 1naterial was laid down in inclosed lake basins, and that the colemanite beds are 

probably the result of the evaporation of Tertiary alkaline lakes during periods of aridity." 

Later Keyes 3 st1ggested a.1nodification of the hypothesis by assuming the Tertiary water 

bodies to have been marine, "a great shallow arm of the Pacific Ocean that had been cut off 

by the upheaval of the mountain ranges along the coast." ·As the fossils obtained from 

the sedin1entary beds associated with the borates, at least in the Mohave Desert, indicate a 

land and fresh-water fauna exclusively, this modification of the hypothesis may probably be 

dismissed.· · · · 

Baker 4 discusses at some length the character of the sediments associated with the borate 

deposits and refers to the genesis of borate ores near Daggett: 

The interbedded layers of colemanite, gypsum, and limestone were most probably deposited on the evaporation of 

a body of water of cot1siderable depth, since the colemanite layer is fr<;>m 5 to 30 feet in thickness. Layers of pure 

gypsum several inches thick are found, as well as more considerable thicknesses of what is probably chemically deposited 

limestone. An alternative hypothesis, that these rninerals had their immediate origin in hot springs and solfataras 

opening diTectly into shallow lakes, perhaps only of seasonal duration, or in playas, has much to commend it, especially 

when considered in connection with the numero~s evidences of shallow-water deposition. These evidences comprise 

ripple marks, sun cmcks, rain prints, found on the finer as well as the coarser beds, and the layers of fine breccia a,ncl 

conglomerates interbedded with the tine shales and tuffs. Shallow lakes or ponds probably existed at times during 

the deposition of the fossiliferous tuff member, for they seem to be necessal'y to account for the presence of the gastropods. The paucity or absence of fossils in the borate and the fine ashy and shaly tuff members (but one specimen of a 

Planorbis was found in these beds), as well as the presence of the colemanite, limestone, and gypsum layer.s, apparently 

indicates the salinity of the waters. * * * Colemanite, gypsum, a11d lime('ltone were deposited either by hot 

springs or solfataras in saline lakes, which might have been of shallow depth, or, having leached from the surrounding 

rocks, were precipitated during a time or times of evaporation of a former fresh-water lake of considerabie depth. 

Lately Eakle 5 has described the colemanite mined at Lang and summarized the origin of 

the deposit as follows: · · 

The bedded character of the deposit is evidence that the mineral crystallized from an evaporating solution, and 

that precipitations of both the borate and some of the silt which formed the shales took place. The solution filled a 

closed basin as a lake or marsh, probably similar to the alkali marshes of the desert regions. It is generally characteristic of such deposits that salts of various kinds, often in alternating series, especially carbonates and sulphates of lime 

and soda, make up the deposit, and the well known Searles borax lake in San Bernardino County, with its many associated minerals, is a good illustration of a desert formation. 'rhe Lang deposit, howev~er, is an exception, as the nco- . 

colemanite is practically unaccompanied by other minenl:ls, except howlite, which is a silico-colemanite, and some 

calcite. \\"aters emptying into the basin could not have been charged with mixed alkali salts. 

It seems probable that· the original site of the deposit was a marsh containing marl and calc tufa with mud and 

considerable organic growth, and that later waters charged with boracic acid flowed into the basin and converted the 

1 Campbell, M. R., Reconnaissance of the borax deposits of Death Valley and Mohave Desert: U. S. Geol. Survey Bull. 200, pp. 8, 12, 1902. 

2 Spurr, J. E., Ore deposits of the Silver Peak quadrangle, Nevada: U. S. Geol. Survey Prof. Paper 55, p. 21, 190G. 

a Keyes, C. R., Borax deposits of the United States: Am. Inst. Miri. Eng. Bull. 34, 1909. 

4 Baker, C. L., Cenozoic history of the Mohave Desert: Univ. California Dept. Geology Bull., vol. 6, No. 15, 1911. · 

5 Eakle, A. S., Neo-colemanite, a variety of colemanite, and how lite from Lang, Los Angeles County, Cal.: Univ. California Dept. Geology Bull.· 

vol. n, No. u, 1911, pp. 179-189. 

L 
THB ORIGIN OF COLEMANITB DBPOSITS. 5 

cu.ruouate of lime into the borate. Some and perhaps th.e greater part of the arg_illaceous material which forms the shales 

was precip:it;atecl by the decomposition of the impure limestone, together with organic Il)atter. The carbon dioxide set 

free may not wholly have escaped, but possibly becam~ occluded in the mud and later converted into carbon. Most 

of the borate is o.f a blackish-gray color, due to iinpregnations of carbon alo.ng the cleavages and fractures. The conversion of the limestone :into the borate in all probability took place-be.fore the overlying sandstones were formed. The 

absence of soda compounds and the presence of abundant plant life indicate that the lake or marsli was fresh, into which 

springs containing boric acid discharged. The deposit later became submerged and the sandstones were laid, down. 

The origin of the boric acid is presumably. volcanic and the springs probably issued from vents in the immediate 

vicinity of the basin, The deposit is situated in a hilly district and is partly surrounded by high masse~. of volcanic 

tuffs and rhyolites. The subsequent tilting of the deposit was not accompanied by heat or pressure sufficient to 

modify the borate materially, yet the mineral shows lines of strain and columnar partings due to pressure and shrinkage. '.I'ho fissile shales owe their solidity to this slight pressure, and carbonization to some extent was also the result. 

There is, of comse, the possibility that all of the carbon in the deposit is from organic matter, the 002 of the carbonate 

escaping, as some of the shales are quite bituminous. 

EnJde does not n1ention any itnn1ediate relation of the deposit to lavas, such as are 

.known to be intimately associated with qorate-bearing beds elsewhere, although he notes 

thn,t the Lang district is partly su1Tounc~ed by high masses of volcanic tuffs and rhyolites. 

Study of the colemanite deposits in Ventun"t County, Cal., and observations on si1nilar 

deposits near Death Valley have led the writer to the conclusion that· these borate 1ninerals 

itnd probably also 1110St of the gypsmn associated with then1 are vein deposits and not the 

product of evaporated lake waters. The evidence supporting this conclusion is set forth 

briefly below. 

COJ.JEMANI'I'E DEPOSITS OF VENTURA COUNTY, CAL. 

The Venturn, County deposits of colmnanite lie on the southern flank of :Mount Pinos, 

netu the San Emigclio Range, at the south end of the San Joaquin Valley. The colemanite is 

found. within a se.ries of bedded rock forn1ations, which have "been extensively folded and 

ft1ulted. In general the trend of their outcrop is northeast and southwest, and the clip southeastward, away frmn the higher elevations of the Mount Pinos Range. 

The section of seclnnentary beds and lava with which the colemanite 1s associated 1s m 

general tenns as follows: 

Section o.(formations in which c9lemanile occurs in Ventura County, Cal. 

Top o[ section measured. Feet. 

Shale aud some sandstone ................................................................... 300 

Basaltic hwa flows, with intercalated layers or lenses of shale and limestone .................... 600 

ShnJe ....... :............................................................................. 600 

Uonglomemte, bowlders; or cobbles o[ light granitic rock cemented ................ : ............ GOO 

Othel' sedimentary rocks belo",.· 

'.l'.hese beds are believed to be of l\1iocene age. The ·u1ore extensive stratigraphie section 

exposed in this general1:egion has not been included here, as its possible relation to the borate. 

ores seems remote and it evidently involves many con1plex p~roblen1s. The beds defined above 

itre apparently confornlttble. The most valuable borate deposits are included within the layers 

of s.hn1n 11nd li1nestone intercalated within the flows of basalt, but borates occur also in the 

shn1cs n.bt)VC t11Hl below the. basaltic :flow rock, though invariably in close association with 

this ln.va. 

· The following interpret~"ttion 1nay be placed· on the section given Rbove. The base of the 

section :is t"t mitssive conglmnerate 1nade up of bowlders or 1nore or less ro~mdecl cobbles; in part 

very :fi.rmly cemented together, and the whole n1ass constitutes a rather distinct fonnational 

unit. Th:is is supposed to be a pied1nont aUuvial deposit that was originally spread out fron1 

the bn,se of th.e Mount Pinos Range. The eore of the range is believed to be 1nainly granite of 

the cluuaeter represented by the bowlders of this forn1ation. The bowlders are coarser near 

the mountains and semn to grade off to finer n1aterial at greater distance, the forn1ation as a 

w.h ole takin·g o.n· the character of sandstone, as illustrated at the I ves camp, at the lower end 

of the ~J.iddle Fork. canyon. There seems little evidence in this formation to indicate other 
6· CONTRIBUTIONS TO GENERAL GEOLOGY, 1913. 

tha1i subaerial distribution of very co~rse detrital material. However, in the opinion of R. W_. 

Pack, who has studied the areal geology of the north slope of the San En1igdio Range, there 

may exist some roon1 for doubt that the coa:r:se gravels were deposited subaerially. On the 

north slope of that range about 10 miles north of the borax mines he has found coarse conglomerate beds containing granitic bowlders several feet in dian1eter interstratifiecl with fine 

sandstones and sandy and diatomaceous 'shales, the latter containing marine fossils of Miocene 

age. The coarse beds occur at several horizons and are so intimately associated with the fossiliferous strata as to suggest that the whole section is n1arine. 

The shale overlying the bowlder conglmnerate and sandstone is light-colored detrital nu-..terial, thin bedded, and so readily eroded that it does not forn1 conspicuous exposures. It has 

not been examined in detail, but it is supposed to be made up of muds derived by the more 

cmnplete co1n1ninution of granitic material similar to the granite in con1position. It n1ay be 

a delta plain or alluvial deposit distributed by quieter ·wate'rs than those which carried out the 

bowlder wash, or it may have been deposited under water. At present there is no conclusive 

evidence as to its origin. · · 

Overlying the shale is a 1nassive accun1tllation of flows of basaltic lava, with vesicular 

and slaggy layers. Evidently many distinct flows are represented in the thickness of some 600 

feet n1easurecl ·on North Fork. These flows include several intercalated layers of shale and 

limestmie, with which the n1ost valuable colmnanite depos~ts of the district are associated. 

The basalt is overlain by another mass of shale and sandstone similar to that which underlies it. The history represented by these beds may be n1uch like that which preceded the 

outpourings of the lava. . 

The colemanite of Ventura County is evidently typical of the mineral as it occurs elsC\vherc. 

It is found in solid crystalline masses of large size but of very irregular fonn. The mineral 

itself in its purest form is milky white, even glassy in part, n1uch resembling pure-white calcite 

in general appearance and cleavage. The crystal forms are, however, quite . distinct from 

those of calcite. The less pure massive ore includes grayish or even black crystalline bodies, 

the color of which is supposed to be clue to included impurity. 

The mineral masses as a whole are of very irregular form, but they have been generally 

described as bedded deposits, for they appear to follow the bedding of the sedin1entary strata 

·with which they are associated. When examined in detail, however, the 1nineral colmnanite 

does not exhibit any bedding structure but only the forn1s cha.racteristic of the crystallization 

of the mineral itself. The massive deposits are either crystalline without deiinite arrangen1ent 

or show radial structures as well as sean1s of definitely vein-banded n1aterial, included between 

walls, in places containing open cavities lined ~rith crystal tern1inations. The irregularity or 

bunchy character of the deposits alone would prohibit strict parallelis1n with the distinctly· 

bedded shales in which they are included. Other evidence afforded by the deposits then1selves 

is suggestive of formation in veins, as will be pointed out. 

The larger deposits of colemanite, such as those of the Colmnbus, Russell, and Frazier 

1nines, are associated with shale and limestone lenses included within the section of basalt lava 

flows. In each of the n1ines 1nentioned the colemanite ore appears to have been developed in 

imn1ediate association with 'a bed of limestone included within the shale. This lin1estone is 

1nassive or of rough, porous character on the weathered outcrop, suggesting the designation 

"travertine-like." Basaltic lava occurs stratigraphically above and belo·w the ore bodies and· 

is generally encountered in the mine workings not far frmu the ore. The shale bands intercalated within the basalt flows 1nuch resmnble the shales that both. overlie and underlie the 

whole sequence of flows. 

The present attitude of the beds that include the ore bodies ranges frmn nearly flnt to 

steeply dipping.· The nearly horizontal ore bodies in the Frazier. 1uine are the developn1en t 

of beds near the crest of an anticlinal fold. In general the rocks dip steeply. · 

In one large deposit examined underground (in the Russell mine) the .main ore body 

follows a zone of slip faulting, which is also approximately parallel to the bedding, the 9re 

being limited with nn1eh distinctness by a wall of slickensided gouge. 
J 

1::i:rE OHIGI.N OF COLEMANI.T:K DEPOSITS. 7 

The outcrops have I:arely shown any colmnanite, even in the ore-bearing zone. The one 

cxcept.ion noted, that of the original discovery at the Frazier 1nine, was an exposure near the top 

of a very steep bluff in which a great mnount of slipping had taken place audnonnal outcrops 

were not present. Possibly .a 1ninor a1nount of the borate 1ninerals is also present elsewhere in 

the outcropping strata, but it is very inconspicuous and was not observed. As a rule the outcrop of the borate-benring beds is either ~olded nnd crmnpled shale or shale inCluding a lllH.SSive 

limestone ledge, the limestone for1ning the n1ost conspicuous pnrt of the outcrop. These zones 

are generally indicated by nn abundance of gypsmn, which is a n1arker followed in prospecting 

and occurs in tl'le form of thin string·ers included i1'1 the operl.ecl bedding planes and in cross fractures 

of the shn.le. All the gypsmn is of the clear crystalline variety selenite, and no 1nassive bedded 

deposits of gypsum are known in tlus region. The outcrops of these gypsi:ferous shales are 

also chn.racteristically 1narked by the occunence of "button" concretions-flattened spheroidal 

nodules or disks of calcareous con1position that have very con1monly been n1istaken for fossils. 

'The occurrence of these concretions at the outcrop of the basaltic lava, in the imn1ediate vicinity 

of the ore-bearing zones, .usually only a few feet distant, is believed to be significant and- is 

genernHy noted and refened to by the prospector. As a rule, however, the prospector regards 

the lava n.s an intrusive "porphyry," which it clearly is not, as attested by its comn1on vesicular 

and n.rnygdn.loidn.l character, as well as its distinctly bedded structure. confonnable with the 

sedimentn.ry s trn. ta. 

Certain specimens collected on the ore clumps and in the mines show that at least a part 

of the colemanite is a replac01nent deposit. Irregulnr portions .of the limestone are surrounded 

by white crystalline colenutnite, and 1ninute fractures which tra~m·se the lin1estone throughout 

n.re n..lso -filled with this rninern.l. These veinlets are observed to have been enlarged irregularly 

within the limestone. Small rounded masses of limestone are also included within the solid 

portions of the colemanite, indicating that in places the san1e enlargmnent of intersecting 

veinlets has been carried to a further stage and the separated limestone portions are residual 

within the deposited colmnanite. 

GJ~NEH.AJ..J .l!JVIDENCE OF VEIN CHAH..ACTER OF COLEMANITE DEPOSITS. 

The direct evidence of the character of the colmnanite, of course, lies in the deposits themselv~s. If the deposits ftTe of vein fo~·m, the evidence suggesting it has hitherto been overlooked. 

In ntttural outcrops relationships are obscure. The study of the ore in place is not facilitated 

by conditions n.t the .mines. The old mining entries on the Ventura County deposits were, when 

visited, either irmccessible or, if open, were-heavily timbered, dripping wi.th water, and coated with 

mud, so that they were not f:1vorable to detailed study of the character of the ore. The facts 

of obsnrvn.tion therefore u.re not yet to be considered conclusive, and judgn1ent as to whether 

these deposits originated as veins or through desiccation involves a study of other more general 

considerations. 

Colemn.nite, the borate of lime, is relatively insoluble in water. A recent determination, 

made at the writer's suggestion. by W. B. Hicks in the Geological Survey laboratory, of the 

solubility of certain specimens of apparently pure colemanite Jron1 the Ventura County deposits 

hn.s shown that t1t ordinary ternperatures (20° to 25° C.) about 1 part is soluble in about 1,100 

parts of water. Van 't lioff has shown that colemanite 1nay be produced artificially by reactions· 

fro.rn other calcium borates in st1turated alkali chloride solutions, but the writer knows of no 

thttt1 concerning the possible formation of this mineral by direct reaction of boric acid and 1imes1iOtH~. It is natural to assume that free boric acid, which is a corl.1mon constituent of the later 

volc11n ie cmn.nn.tions in this general region, might react w1th limestone, :1nd th11t by th~ su b~titution of boric for en,rhonic acid the li1i1e borate would be formed. Thus by a process of metnsonutiiic replitcetnent might h.ave been formed deposits of the typic:tl irregular chn,racter of the 

.known colom1tnite masses, roughly following the beclcling of the original c·alcareous rock or of 

the interbedded lenticular bodies of limestone. Therefore in so far as the natural reactions are 

concerned the :fonnation of colmnanite as fissure and replacement vein deposits is well within 
8 CONTlUB.UTIONS TO GENERAL GEOLOGY, ·1913. 

tl}.e limits of possibility. But for other reasons also~ it seems that these colemanite deposits are 

not logically to be ascribed to deposit~on as the principal constituent of a residue derived from 

the waters of a de~iccating saline lake. 

Borates have been shown to exist in riatural saline waters and desiccation deposits in many 

parts of the Great Basin and the desert country of the western United States. So far as· known 

to the writer, how'ever, c·olemani~e has nowhere been deposited as a result of desiccation, but 

rather the boric acid has combined to form the more common surface saline minerals, such as 

ulexite and tineal. Is it therefore reasonable to suppose that ulexite, tineal, etc., could have 

been the primary deposits of such Tertiary dry-lake formations, from which colen1anite had 

subsequently forn1ed as a secondary product~, If this had been so, would not some evidence 

of the prirnary deposit be at hand, either in the composition or in the structure of some parts 

of the colemanite deposits~ Such evidence has not been recorded. 

If colemanite were but a part of a natural saline residue deposited by the desiccation of the 

waters in which these .materials had accun1ulated and becon1e concentrated, it would be n1ost· 

natural to expect other salines that are commonly n1ore abundant than the salts of boric acid in 

the typiGal· desiccation deposits of the desert region. No appreciable amount of bedded salt, 

sodium sulphate, or carbonate is to be found with the colemanite deposits knqwn to the wr1ter. 

The borate-bearing shales of the Ventura County deposits are n~t even of a character that might 

be described as "alkaline" in ordinary desert parlance. - · 

Gypsum and limestone, the latter travertine-like, occur in association with the borates, 

and former writings have already been quoted to the effect that both are chemical residues 

derived from the same desiccation that crystallized the colemanite. The gypsum associated 

with the borate-bearing shales is crystalline selenite. All or the greater part of it occurs in thin 

stringers, which not only follow the bedding joints of the shales but fill transverse fractures. 

The cross~ fiber structure of much of this selenite is evidence of its deposition in veins. No rock 

gypsinn of distinctively sedimentary character was observed in any of these be'cls. Gypsum is 

being deposited frmn ground waters along the walls of the mining entries at the present tin1e. 

It still remains to .be proved, therefore, that any gypsum of original sedimentary character is 

included with the borate-bearing deposits. All of this mineral present may have been introduced in solution by percolating ground waters. The stringers of gypsum noted so abundantly 

1n the outcrops of the borate-bearing shales have. not been shown to persist in like quantity in 

the few deeper developments of the mining entries. 

The limestone strata· with which the princip_al colemanite ore bodies are associated are 

evidently original in the sedimentary sequence. They are in part at least trav.ertine-like, 

weathering in rough tufa-like surfaces, and are not compact like more typical limestones. Their 

character suggests the probability that they are chemical deposits, possibly of local extent, laid 

down in shallow waters, being similar to the travertine deposits now forming hear springs or 

where ground waters flow into ponds or saline lakes. The limestone masses are believed to be 

of lenticular form and to occur interbedded with shales at various horizons within the flows of 

basaltic lava. They are not necessarily the p~oduct of desiccation. 

In fact, there is little evide:p.ce from the s.tratigraphic section associated' with the borate 

beds of Ventura County that can be taken to indicate the extensive lacustral conditions comJnonly assumed to have existed during the :Miocene epoch in the Tertiary. Some of the associated deposits may have been "lake beds," but the writer believes rather that the submergences 

· were the result of local, temporary, and variable condition~, such as may have occurred during 

or after periods of shallow flooding in the lower portions of piedmont fluviatile plains. . 

Lastly, an explanation of the source of the boric acid is to be sought. Naturally, a volcanic. 

origin is suggested, and in Ventura County the association of the colemanite with the basaltic 

lava is certainly intimate. It is a common saying an1ong prospectors of the.district that" borax" 

will not be found except near a '~porphyry contact" and in association with limestone. It is. 

true that the larger ore bodies mined are all in close relation with some part of the basaltic lava 

flow rock in place, generally,within a few feet of it~ The borate-bearing beds follow the .outcrop 
THE ORIGIN OF C,OLEMANITE DEPOSI'J:~. 9 

of the basalt flows throughout the district. The question in what way boric acid originating 

with the extrusion of these flows could have caused reactions and mineral deposition soon or · 

long after the period of the lava extrusion remains for further investigation. · 

SUMMARY. 

'T'he evidence favoring the hypothesis of a desiccated saline lake to explain the origin of the 

colCJnanite has little to support it beyond rather general assumptions. The actual character of 

the deposits then1selves indicates ·rather a vein type of formation. The gypsum which has been 

pointed to as a desiccation deposit related to the colemanite is also of vein character. Other 

salines which would naturally be expected in desiccation deposits resulting from natural saline 

solutions are not found in association with the colen1anite. Those who have s'i.1pported the 

des.ic~ation hypothesis have offered no explanation of the reaction which might produce colenuwite in such 1nassive deposits as a product of water evaporation, while, on the contrary, its 

for1nation fron1 li1nestone in veins by replacCJnent of carbonic acid with boric acid is a natural 

working hypothesis that deserves experin1ental investigation. The relations of the deposits to 

basalt lava flows indicate the probable origin of the boric acid at the time of the extrusion of 

these lavas, althotigh it Inay also be necessary to assume that this acid continued to find its way 

into solution of the circulating ground waters long after the period of the lava extrusions. 

0