56 
AMERICAN GEOLOGY. 
supposed chloritic compound forms chloritic slate in combina¬ 
tion with quartz, in which respect it is analogous to talcose 
slate. Its composition is as follows: 
Chlorite. 
Silex, 
26-0 
Alumina, 
18-5 
Magnesia, 
S‘0 
Soda, 
2-0 
Oxide of iron. 
43-0 
Chromate or chromite of iron occurs in serpentine, but where 
calcspar or carbonate of lime is associated with serpentine, it is 
often absent, as in northern New York and New Jersey. Octa¬ 
hedral iron, as well as the specular iron, is often associated with 
serpentine. 
§ 45. Hornblende group. The associates of hornblende are 
feldspar, pyroxene, and quartz. It sometimes stands by itself, 
or has no intermixture of feldspar or quartz. But generally 
feldspar is present, and the two minerals are arranged in 
parallel stripes, appearing like a stratified rock. Quartz is 
never abundant in this compound. 
Hornblende is associated with feldspar in another class of 
rocks—the pyroplastic, the greenstones, or as they are some¬ 
times called, dolerites and diorites. Their structure is more or 
less granular, and the feldspar may be seen in white crystal¬ 
line grains, sometimes by the naked eye, but in many instances 
the eye requires the aid of a magnifying glass. It appears 
that the hornblende in these compounds is greater than that of 
feldspar. Sometimes again the feldspar is in quite large 
crystals, giving the rock a spotted appearance. It has become 
a greenstone porphyry. This is the case in many places situ¬ 
ated in the outburst of greenstone along the Hudson and 
Connecticut rivers. 
Chemists have paid but little attention to the composition of 
rocks, and hence it is impossible to group them as perfectly as 
it is wished. Our trap dykes, which are usually so homoge¬ 
neous and compact, afford no external clue to their composition. 
They may be hornblendic, or they may be pyroxenic. The 
