Semi-weekly herald., 1913-03-24
ENGINEER TELLS OF THE
ORIGIN OF LA PLATAS

The main igneous rocks as far as
determined are divided into five diff
ferent classes: Trachyte, diorite, mon
zonite, syenite and andesite in the
La Plata district.
From all indications there was at
an early period one action after an
other, covering an area of ten square
miles, splitting the earth's crust, the
old primitive rock, the granite and
sedimentary rocks in every direction,
forming fissures and stringers some Of
which extend for miles from the main
| upheaval. The first action, the ira
chyte, which is not visible, only in a
very small spot in the northwest part
of the La Plata district, as the ma
jority Of the district has been COW
cred with volcanic matter of latter
period.
Second, the diorite, which resembles
granite, consisting of hornblend and
albite, a greyish white to nearly black
in color. 1t derives its name from
being clearly defined as distinguished
from dolorite. This action predomi-
mates in the north and western por-
tions of the district and shows no
values only where assures were form
ed by later actions.
The monzonites, evidently the third
action, was first found in Mount Mon
zoni in the Tyrol, hence derived its
name, the composition which is silica
alumnia protoxide of iron magnesia.
lime, soda and potash. This volcanic
matter has been found in veins and
r,nglets through the entire district and
at the point of action covers the sur
face by the overflow, but does not car
liy the agents for precipitation of the
precious metals.
Syenite porphyry, the fourth action
in this district, which principally con-
sists of orthoclass feldspar and horn
blend; when found in assures contains
iron which has precipitated copper
forming native copper, cuprite, mala-
chite, azurite, calcopyrite and erybe-
scite, with small values Of gold and
silver in chloridized form. This class
of porphyry does not predominate
yet large veins with their tributaries
are found in the district west Of the
La Plata river in the copper belt, but
diminishes on their strike east
Andesite, the last action, as far as
determined in this district, is very
similar in composition to monzonite
but has double the hardness and COM
pressed to much finer grain by heat
and pressure, is otter altered ano
changed to kalinite. 1t derives its
name from the Andes, where found
Color, white, gray, greenish, yellow
ish or flesh red, and lustre subritreous
inclining TO pearly.
This action is found only in small
veins and reinlets in any part Of thu
La Plata mountains, but is the mali
| action that deposited the rich depos
its and when found contains grea'
values. These actions are the source
of all minerals, without the volcanic
| action no igneous rocks would be pres
ent, no ore would be found unless
carried by some glacier action from
some unknown volcanic region and de
, posted in blanket form in the sedi
mentary. The La Tata mountains
have been blesseZ with not only on
volcano but many, and at differen
periods. There is no district tina
shows more disturbance.
Il Evidently at the time o! action tn
, earth's crust, the old primitive gran
ite, was very thin and the carboni
, gas with the molten matter must leav
out, and this being a weak point
burst forth with a vengeance, split
ting the earth for miles around. A
for example an old crater would re
semble tree. Taking the body o
a tree as the center Of volcano
the main limbs and small branche
, representing stringers in mines as tn
limbs are large at body of tree an
, diminish away from it, so are tn
, veins, they gradually get smaller un
1 til no trace of them can be founc
! after running for miles from the mai
! action. The old craters and the mai
, limbs are the only true veins an
original deposits of ore, they ar
i called dykes Of phorphyry which the
are but Of which class do they be
, long must be considered.
Then comes the secondary deposit
, of minerals in the sedimentary for
;. nation in all forms, flat and infille
: gashes. The gashes are not connect
ed in any way with the main actor
: They are simply cracks in the sedi
I mentary CAUSED by the earth's crus
y expanding to give vent to the case
' and eruptive matter. The erosion
: either by giseisr action or water, an
possibly both, dsposited and oxidize
and chloridized minerals in crevice
\ and low flat places, coming in contae
: with some precipitatn and silica while