PYROCRYSTALLINE ROCKS. 
55 
formed at different periods. It is associated with bronzite, dial¬ 
lage, talc, rensselaerite, schillerspar, steatite, chromite of iron, 
chalcedony, and calcspar. It will be perceived that the group¬ 
ing of serpentine differs from other pyrocrystalline or pyroplastic 
rocks. Serpentine itself is composed of 
Silex, 
Serpentine. 
42-50 
Magnesia, 
38 63 
Alumina, 
1-00 
Oxide of iron, 
1-50 
do. chrome, 
0-54 
Lime, 
0-25 
Water, 
15-00 
The associates of serpentine contain magnesia in different 
proportions; thus bronzite and diallage: 
Bronzite. 
Diallage. 
Silex, 
60-00 
52-00 
Magnesia, 
27-50 
15-91 
Alumina, 
3-18 
Oxide of iron, 
10-50 
7-47 
Lime, 
19-59 
Diallage is also associated with feldspar, and forms with it 
diallage rock. In this country, however, it is a rare rock. 
Steatite and talc contain magnesia, but less iron than the fore¬ 
going. They are found to be composed of 
Talc. 
Steatite. 
Silex, 
62-00 
4S-3 
Magnesia, 
27-00 
26-65 
Oxide of iron, 
3-50 
2-00 
Alumina, 
1-50 
6*18 
Water, 
6-00 
9-05 
The soapy bed of talc and steatite is characteristic of these 
minerals. They scarcely differ, as will be seen by the foregoing. 
Chlorite, a deep green mineral, which often looks and feels like 
a green talc, differs however in composition. Chlorite is some¬ 
what important as a mineral species; it is usually associated 
or grouped with quartz, particularly with milky quartz, as in a 
part of the Taconic range of mountains in Berkshire, Mass. A 
