DESCRIPTIONS OF ROCKS. 423 
2. The distinction of a triclinic feldspar from orthoclase, 
the former showing in sections, cut in any direction ex- 
cepting one, commonly several parallel spectrum bands, due 
to multiple twinning in the crystal, while orthoclase shows 
no bands of the kind, or at the most but two. 
3. The presence or not of hornblende ; this mineral hay- 
ing often cleavage lines meeting at angles of 124°, and being 
dichrote. 
4. ‘The presence or not of pyroxene; this mincral often 
showing cleavage lines meeting at angles of 87° (nearly a 
right angle), and being not dichroic, and usually distin- 
guished in this way from hornblende. 
od. The presence or not of mica, its cleavage lines and 
dichroism affording distinctive characters. 
6. The presence or not of chrysolite ; of magnetite, its 
form being often octahedral, and single or grouped; of 
9. 11. 


Magnetite in grouped Liquid Carbonic Cube of Salt in a solu- 
crystals. Acid. tion of the same. 
points or portions having the nature of glass, and therefore 
not polarizing light; of fluidal lines; of liquid carbonic acid, 
and of various other zclusions. Fig. 9 shows a common 
form of the grouping of microscopic magnetite crystals 
in an eruptive rock. Fig. 10 represents a cavity in quartz 
nearly filled with a liquid, 2—the small bubble, c, showing 
the part not occupied by it. When the liquid is carbonic 
acid the air-bubble disappears on raising the temperature to 
86°-95° F. Carbonic acid requires a pressure, at 32° F., of 
383 atmospheres to retain it in the liquid state ; and hence 
occurs liquid only in quartz, topaz, and a few other miner- 
als. Fig. 11 (from Zirkel) shows another cavity, containing, 
besides a liquid, a little cube and microscopic hornblende- 
like acicular crystals; and the cube is supposed to be com- 
mon salt in a solution of salt. Hexagonal prisms of apa- 
tite (calcium phosphate) are detected by the microscope in 
