DESCRIPTIONS OF ROCKS. 423 



2. The distinction of a triclinic feldspar from orthoclase, 

 the former showing in sections, cat 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 hav- 

 ing often cleavage lines meeting at angles of 124°, and being 

 dichroic. 



4. The presence or not of pyroxene ; this mineral 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. 



o. The presence or not of mica, its cleavage lines and 

 dichroism affording distinctive characters. 



G. The presence or not of chrysolite ; of magnetite, its 

 form being often octahedral, and single or grouped ; of 



11. 



Magnetite in grouped Liquid Carbonic Cube of Salt in a solu- 



crjstals. 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 inclusions. 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, o — 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 

 38-J- 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 



