THE NATURAL HISTORY SURVEY. 



49 



Fracture earthy. Hardness, i. Specific gravity, 2.5. Color, 

 white, reddish, yellowish, greenish. The feel is that which is 

 called meager. It is plastic, being readily shaped when moist, 

 and retaining its shape. Before the blowpipe it is infusible. It is 

 slightly soluble in concentrated hydrochloric or sulphuric acid. 

 It results from the decomposition of feldspar. The mixture of 

 kaolinite with fragments of a great variety of rocks forms the 

 material known as clay, of which we have such an extensive 

 supply in this region. 



PHOSPHATES. 



Apatite. 



j (Ca, F)Ca 4 (P0 4 ) a . 

 ( (Ca, Cl)Ca 4 (P0 4 V 



Hexagonal. 



Fig. 19. 



Crystals of apatite are common in argillaceous or in igne- 

 ous and metamorphic rocks, and they occur also in sediment- 

 aries. The planes most often seen are the prism (iolo), modified 

 by pyramids (loll) and by secondary pyramids (1121). The 

 crystals are usually long thin needles, showing a parting trans- 

 verse to the c axis; that is, parallel to the base. - Fracture, 

 conchoidal and uneven. Tenacity, brittle. Hardness, 5; 

 specific gravity, 3.17; lustre, vitreous to subresinous; streak, 

 white; color, sea green, bluish green, violet blue, yellow, red 

 and brown. Transparent to opaque. Under the microscope 

 colored varieties show slight dichroism. The moderate re- 

 fraction of apatite gives, under the microscope, but a medium 

 relief. Its double refraction is very low, and the axial cross 

 can be seen in convergent light with difficulty. The interfer- 

 ence colors are dull, not above grey of the first order. The 

 columnar crystals show parallel extinction, and are broken at 

 right angles to the c axis. Apatite colors the Bunsen flame 

 reddish yellow, showing calcium; and when moistened by sul- 

 phuric acid, a pale bluish green results from the phosphoric 

 acid present. It dissolves in hydrochloric and nitric acid, but 



