
“a -Ss i : 
_ oR ee We - re 
‘Parr Il.gii] | ROCK-FORMING MINERALS. 15 
forming entire masses of. rock (hornblende-rock, hornblende-schist), 
are the common forms in granitic and yolcanic rocks (granite, syenite, 
diorite, andesite, &c.) The former group naturally gives rise by 
weathering to various hydrous magnesian silicates, notably to 
serpentine and tale. In the weathering of the aluminous varieties, 
silica, lime, magnesia and a portion of the alkalies are removed with 
conversion of part of the earths and the iron into carbonates. The 
further oxidation of the ferrous carbonate is shown by the yellow and 
brown crust so commonly to be seen on the surface or penetrating 
cracks in the hornblende. The change proceeds until a mere 
internal kernel of unaltered mineral remains, or until the whole has 
been converted into a ferruginous clay. 
Augite (Pyroxene). Monoclinic, chiefly in short stout prisms ; also 
granular, more rarely lamellar or fibrous ; ranging from white through 
shades of green to black. H. 5—6. Gr. 2-°88—8'5. Divided like horn- 
blende into two groups. Ist. Non-aluminous, CaMg 2810, con- 
sisting of meta-silicates of magnesium and calcium (silica 49 to 56 

Fie. D.—SECTION OF AN AvciTE COrysTaL FROM A BASALT-DYKE, CRAWFORDJOHN, 
LANARKSHIRE, MAGNIFIED, SHOWING LINES OF GROWTH, WITH VESICLES, AND Mac- 
NETITE CRYSTALS, 
per cent.), usually with a little protosilicate of iron (very commonly 
also of manganese), which gives the prevalent green colour to the 
group (malacolite, sahlite, &c.). 2nd. Aluminous (silica 47 to 55 per 
cent., alumina 4 to 9 per cent., with a small variable proportion of 
ferrous oxide), including generally the dark green or black varieties 
(common augite, fassaite). It would appear that the substance of 
hornblende and augite is dimorphous, for the experiments of 
Berthier, Mitscherlich and G. Rose showed that hornblende, when 
melted and allowed to cool, assumed the crystalline form of augite. 
Whence it has been inferred that hornblende is the result of very 
slow, and augite of comparatively rapid cooling, 
Under the microscope augite in thin slices is only very feebly pleo- 
ehroic, and presents cleavage lines intersecting at an angle of 87° 5 
