
ROCKS 89 
likewise soft, pale white or greenish, and with only a slightly soapy feel. 
A rock composed of a fine-grained schistose aggregate of dark green or 
greenish-black fibrous scales is probably ornblende-schist; a similar 
aggregate of dark or light green acicular or ray-like fibrous crystals is 
probably actinolite-schist. The other common schistose rocks—mica- 
schist and gneiss—should not be hard to distinguish, even in fine-grained 
varieties—seeing that the constituent minerals are all easily recognisable, 
especially on the edges of the folia. 
The Argillaceous, Calcareous, Siliceous, and Felspathic rocks 
undoubtedly form the bulk of the earth’s crust—the other kinds of rock 
not included under one or other of those heads taking quite a subordinate 
place. Many of them, however, are of great economic importance— 
conspicuous members of the series being the coals, carbonaceous com- 
pounds of all kinds, ironstones, and various minerals, which now and 
again play the part of rocks. Among the latter are rock-salt, gypsum, 
anhydrite, apatite, etc., but as the distinguishing characters of the 
minerals have already been given, they need not be repeated here. If 
the student can recognise the minerals in small specimens, he should not 
have much difficulty in diagnosing them when they appear as massive 
aggregates. 
The specific gravity of rocks is not infrequently a character of some 
importance, and of no little assistance in their determination. The 
following table gives the average specific gravity of a number of 
representative igneous rocks * :— 
Granite : s : 2-6—2-7 , Diorite ; : 2-7—2-9 
Quartz-porphyry . : 2-4—2-6 | Andesite . < 2 2-4—2-7 
Rhyolite . ; : 2:-4—2-5 | Gabbro ‘ : : 2-7—3-0 
Obsidian . , : 2-0—2-3 | Dolerite : : 2 2-7—3-0 
Pitchstone . : : 2:3—2-4 | Basalt. : : : 2-8—3-1 
Syenite , : : 2-6—2-9 | Nepheline-basalt : 2-9—3-0 
Orthoclase-porphyry . 2-6—2-7 | Leucite-basalt . 2-8—2-9 
Trachyte . : 2 2-4—2-6 | Limburgite . ; : 2-8—3-0 
Phonolite . : , 2-5—2-6 | Peridotites . : : 3-0—3°5 

* The student who desires to take the specific gravity of a rock 
cannot do better than employ the simple and satisfactory instrument 
described in Appendix C. 
