310 The American Geologist. November, 1894 
rounded grains, either scattered through the rock or collected 
into aggregates. It does not possess well-developed crystal 
planes. The pyroxene has very frequently altered to serpen- 
tine. Limestone is often found, hand-specimens of which are 
speckled with small greenish rounded masses of serpentine. 
Under the microscope these greenish masses are seen to be 
aggregates of serpentine, generally associated with granular 
magnetite and sometimes other decomposition products. 
Often, however, undecomposed fragments of pyroxene are 
seen embedded in the midst of the serpentine aggregate, show- 
ing that the serpentine has originated in the decomposition of 
the pyroxene. When serpentine occurs in the white lime- 
stone it is generally in such small masses, derived from the 
alteration of isolated grains of pyroxene. 
Brown biotite occurs frequently in the limestone in large 
rounded grains, often beautifully fresh. Green monoclinic 
hornblende occurs, sometimes very abundantly. It is dark 
green in color and is present in irregular grains in the basic 
concretions in the limestone. Quartz is abundant. Apatite 
is frequently present in irregular grains. Tourmaline occurs 
rarely. Graphite was sometimes seen in the limestone in the 
field, but is not a widespread mineral. In one or two outcrops 
considerable quantities of red garnet were seen in the lime- 
stone. 
In some places quartz is an abundant accessory mineral in 
the limestone. It usually occurs in small grains or masses, 
and the limestone then has the appearance of holding small 
pebbles of quartz. Sometimes these pebble-like masses of 
quartz are an inch or more in diameter. Under the microscope 
the quartz of these masses is seen to occur in very irregular 
grains, with a wavy extinction, and having the appearance of 
vein quartz. There is nothing to suggest that they may have 
had a detrital origin. They are probably metamorphic. 
There are two special types or varieties of rock associated 
with the white crystalline limestones of Jenny Jump moun- 
tain which call for special mention. These varieties are : — 
(1) Pyroxene-rock. 
(2) Quartz-rock. 
The typical pyroxene-rock consists whollj' of irregular or 
rounded grains of colorless monoclinic pyroxene. Very fre- 
