yew Jersey Limestones. — Westgate. 309 
ance the rock locally takes on other colors. With regard to 
the chemical composition of these limestones the writer has 
nothing new to offer. The crystalline limestones in other 
parts of the state show all gradations from a pure lime-car- 
bonate to a typical magnesian limestone, and the rocks of the 
area under consideration probabh T have much variation in the 
amount of magnesia present. Below is an analj'sis* of a 
sample of crystalline limestone from the east side of Jenny 
Jump mountain : — 
Lime 42.45 
Magnesia 10.23 
Oxide of iron and alumina 1.00 
Carbonic acid 44. <>7 
Silicic acid and quartz 1.95 
This indicates an intermediate rock between a pure limestone 
and a magnesian limestone. 
Lithologically the rock varies greatly, but mainly in the 
presence and character of the accessory minerals. Crystal- 
line calcite is usually the principal and sometimes the only 
mineral present. The rock is then composed of an aggregate 
of irregular grains of calcite. The rock is rarely, however, a 
pure limestone, but almost always carries a greater or less 
amount of accessory minerals, and it is with these that this 
paper is chiefly concerned. In the field the limestone is often 
seen to be streaked with greenish or black irregular masses or 
concretions. When examined under the microscope these are 
seen to be composed mainly of pyroxene, hornblende, magne- 
tite and biotite. Other accessory minerals are often present 
in the limestone, such as quartz and, more rarely, tourmaline 
and apatite. Graphite and garnet were seen in the rock in 
certain places in the field, but were not present in any of the 
slides examined. 
Pyroxene is perhaps the most abundant of these accessory 
minerals, and occurs in greenish patches in the limestone. 
Under the microscope it is monoclinic, has a well-developed 
prismatic cleavage, is colorless or very pale green, and non- 
pleochroic. It is probably closely related to malacolite, which 
agrees with it in optical properties and place of occurrence. 
It occurs in large, sometimes irregular, but more generally 
*Geologyof New Jersey, Cook, 1868, p. M)2. 
