New-York tribune., 1874-08-18
THE IDENTIFICATION OF ROCKS AND IFS
GEOLOGICAL SIGNIFICANCE.
HY PNOF. Z. srLEkY HrNT or BOSTON.

This subject the speaker had noticed briefly
IN communication to no Association Lac; year the
geology OF the Blue Ridge. The change OF the rocks in
question Is chemical which II most obvious m tho
case of crystalline rocks the feldsplr loses its alkcs
aud part ofts silver, being changed into clay, ana thc
hornbende its lime and magnesd, retaining us iron aud
peroxyde From tbs resdlts afsof,enng ana decay
greater or less depths Of the strata tnat while the
beds SIN retain their rau-emea and are seen to b5
r.ycrsCd By veins OF quIrz AMI metallic ores, the s'ra,a
are o'ten 20 much chansedio 10 depths OF oae hdldred
feet or more from the surface 20 be readily ronov-d
by the action OF tue water ThiS phenomenon u wei
sen III the crystalline rock, Of the Bue Ridge and not
les remarkably ID those Of BrazI whrto u has ".'A
UoucoI by many observers, among the latest OF wn'L
is Prof IIlrtt. Darwin, pho long ago described a.,
imagined the change TO nave born tf."ted beneath tuc
sc.; but accOrting 10 the speaker II mas been sab
a"a process, which has been al work during past az
when the composition OF the sphere and the cli-
cybOu.c cld, aided Dl waru;aud moisture, aboundod.
He connected II wth that slow publication Of
been going The aikaIes andliue and magnesla ret
tree 10 th process absorbed the atmospheric cdrDJnc
mcstOucs, dO!Jnltes, and sea salt such process
OF decay was already in ae;on at an early period aud
from facts observed by Puupelly IN Missouri had al
tected the r0nJearng iedspar porpayrcs 1T the com
InenstmCh' oI We pdicO|Ols te. Was hecorGihI 10
the SPEAKER washing down of the thus dccOui-
crystalline rocks tIat all the sluys aud sands
which had gone 10 Duild up the sediments U vast
beds OF pacozoc more recent rocks hai been de-
rived lie oIgh II probable that the process Of decay
had gone with decreasing energy to our times,
though II Is now insignificant II lts action OWN. to
chan Scd atmospheric conditions.
The speaker drew picture OF North America past
geological ages. The frequently taught notion OF the
growth OF continent southward aud outward from
nucleus the vicinity Of the great lakes has founGa
fact, and the study OF the unCrysaiine sedi-
mentary formation tells "a different story. The great
palcoZOlc basin was ttihe east aud west well
NORTH surrounded by acres oi decayug crystalline
rocks. Those ot Ncv-E1gidnd. the Blue Ridge
ana the crystalline rocks to The east H. REMAINS
OF great disintcrrated and wasted continent whose
Dut up the uucrYstailIne rocks west
ward, well as those along the eastern aud southern
borders OF the UhIied States Up a comparatively re-
period the hills Ot New England, Eastern New
York and NeWJcfsey were probably like those further
south along the Bue Ridge, deeply covered by the
products OF their decay aad from thcse were dc.
rived tertiary cays. as well as the brown iron ores
which found along the base OF the Blue Ridge
Northern cJntiuuatISn 1t during the glacial
and subsequent gTadui uplift
eastern America, at whlob hne II exposed to the
action oi local glaciers aud 10 tue iceberg of the
polar current, that the nnal removal OF
covering from nils lad taken place wnie turlbrr
southwark the mountains beyond tue reach of mi. de-
nuding action stl retain to-day their covering of de
caycd rock similar condition OF things seen 10 the
northwest In Minnesota, where, ACCORDING 10 Prof.
W1te, the decayed cry sdliues nave escaped denuda
ton.
Tue process OF decay In the more and granite
like rocks had often been incomplete and. working from
natural oints had left unchanged nuclei OF hard rock
uasses bOwlders point which has been well brought
out Mr. BurDank from MS studies In North Carolina
ana throws much light our northern drft posits
A profound decay also an octcd the haId paIeo.
zoc inesOnes whicn the carbonate OF lime has
been dissolved leaving porous rotten rock behind
This Dawson has shown, Is well In the Impure
ar,1llaceOus Trenton limestone near Montreal which ia
localities protected by trappeau dykes from the eroding
which comes from the northeast Is found deeply
decayed while elsewhere, near by, a hard surface 10
worn down md gacaied.