DESCRIPTIONS OF ROCKS. 411 
different molecular nature ; in other words, to saussurite. 
Some of the material appears to be still dabradorite. 
¢ The iron-bearing minerals, Lpidote (p. 262), Garnet 
(p. 296), 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- 
clum-magnesium carbonate (p. 219), which in its rock-form 
1s 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 ciliceous 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-atumina 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 sodalite, afford much soda, 
the first two usually 8 to 12 per cent., and sodalite, 20 to 
25 per cent. Nephelite (elzolite) 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 dit- 
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 
Coe im composition with those in crystals or in massive 
orms. 
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, 
