THE
McCarthy weekly news
Entered as second class matter. Septem
ber 17t,h, 1918 at the l ost Office, a
McCarthy. Alaska, under the Act o
March 3. 1879.
C. O. Marwood Editor
K. Marwootl Circulation Manager
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Saturday, August 29, 1925
P0RPHR1ES
by C. H. S
This article is written
from a practical point
of view and deals with
the physical properties
and mineral composition
of the McCarthy porphry
together with a word or
two of its relation to
ore deposits.
Prospectors and others inter
ested in mineialsare frequently
under the impression that “Pen
phry” implies the nane of a del
inite igneous (molten) rock. When
we speak of porphry we first ha
ve m mind its texture. By text
me we mean the size of the cry
stal grains ami their shape, col
or, etc.
AiJ igneous rucks, rocks cool
edt from a molt*. n mass, have
three physical characteristics
with reference to its texture or
crystallinity; they uie gra.i.eu
uense o'r glassy. We recollect
granite has an even granular
textu re—tn.it is the crystals are
about the same size, but not the
same shape since the miner; Is
which compose granite - qua, tz,
feldspar, the micas and inn mb.
lende have their o.\n shape in j
crystallizing out.
Porphries lack the even gra
ined texture of granite and sim
ilar rocks. They always s-how a
sharp contrast in size between
the crystal grains and the mate
rial in which they lie imbedded,
called the ground mass.
Porphries are classified in
two main groups, one in which
both the crystals and ground
mass can be determined, and the
other in which some crystals
may be seen in a baonground
impossible to determine. It shou
Id be remembered lie re that all
gradations exist for we find one
group merging into the other
Experience is necessary to class
ify them correctly.
Another classification is
made on its chemical cum posit
ion. One group are the lighter
colored in which quartz and leld
spar predominate ; in the other
group the dark iron magnesium
minerals aremore abundant than
quartz and feldspar. Here^again
we iind rocks which are transit
ions between trie two groups.
hVn-phries are named alter the
mineral content. To simply say
■‘tWphry” is insufficient. For
instance we have granite, syenite
thorite porphry, alter the pred
ominating mineral. The MeCar
thy aistnct porphry, which is
familiar to most oi us, is a quartz
uioriie; quartz, b ‘cause it cont
jams tree* quartz and Uioriie, bee
ause a uioiite is a soda-lime feld
spar and hormbiende in about
equal amounts, it is a porphry
because tne rodu contains cryst
aisof free quartz and darkgieen,
neeuTe like crystals oi liormbien
de imbedded in a ground mass
oi tbe same compounds with the
feldspar. With regards to mineral
compounds there seems not aio
lie sufficient silica to have com
hired wiih soda, lime, aluminum,
etc. to form tha feldspar and
hormblonde but a slight balance
winch had to crystallize out as
free quar.z.
Grains of m gnetite, the mag
retie oxide of non, are present.
The theoritic origin of por
phries is interesting. Granite and
similar‘locks are held to have
had time and heat to cr,\ stall
ize out fully, hence its even tex
continued on page 3
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