DESCRIPTIONS OF ROCKS. 41 1 



different molecular nature ; in other words, to saussurite. 

 Some of the material appears to be still labradorite. 



7. The iron-bearing minerals, Ejridote (p. 2Q'Z), Garyiet 

 (p. 25G), Chrysolite (p. 255), which characterize some varie- 

 ties of rocks. 



B. The calcareous species are calcite or calcium carbonate 

 (p. 215), in various states of impurity ; and dolomite or cal- 

 cium-magnesium carbonate (p. 219), which in its rock-form 

 is undistinguishable in external aspect from calcite. 



Gypsum, or hydrous calcium sulphate, is also a consti- 

 tuent of beds among rocks, and should have its place in the 

 list, although not strictly embraced under the term calca- 

 reous. 



Of the siliceous minerals, orthoclase (with microcline), 

 and the two micas, muscovite and biotite, are related in com- 

 position, in that each affords 10 per cent, or more oi pot- 

 ash. Leucite is another allied potash-alumina silicate, even 

 richer in potash than orthoclase, it containing 17 to 21 per 

 cent. The rocks characterized by these minerals are hence 

 rich in potash. 



Albite and oligoclase, and also socialite, afford much soda, 

 the first two usually 8 to 12 per cent., and sodalite, 20 to 

 25 per cent. Nephelite (elseolite) is also a soda mineral 

 related to the feldspars ; but, with 15 to 16 per cent, of soda, 

 there are 5 or 6 of potash ; rarely the alkali afforded is all 

 soda. 



The ordinary kinds of hornblende and pyroxene, on the 

 contrary, afford little or no soda or potash. They thus dif- 

 fer widely from the potash and soda species just mentioned, 

 and naturally characterize for the most part a distinct series 

 of rocks. 



Much importance has been allowed in lithology to the 

 distinction of foliated under the species hornblende and 

 pyroxene ; when, in fact, neither in mineralogy, as all 

 treatises admit, nor in lithology, has it more than a very 

 subordinate value. The character obtained this distinction 

 before it was fully understood that the foliated forms were 

 identical in composition with those in crystals or in massive 

 forms. 



Hornblende does not differ from augite in composition ; 

 but since the difference in crystallization is connected with 

 a difference in the physical conditions attending their origin, 



