Minerals of Rocks. 
53 
3- MICA —The shining scales found in river sands are 
usually one of the micas. All micas have a very 
perfect cleavage. The thin leaves or plates of 
mica are flexible and elastic. A plate of mica 
shows its elastic properties when slightly bent, as 
it springs back to its former position. Plates 
and scales of Talc, which somewhat resemble 
mica, show no elasticity, and when slightly bent 
remain bent. The two more important varieties 
of mica are Muscovite and Biotite. Muscovite is 
usually of a silvery-white color, while Biotite is 
usually black or dark green. Partly decomposed 
Fio. 14.— Microscopic section showing flow-structure of lath 
shaped Felspars in Basalt. 
and altered Muscovite, such as is found in river 
sands, is of a golden yellow color. 
4. HORNBLENDE. — Hornblende is one of the con 
stituents of Diorite, and an important rock- 
forming mineral. It is nearly, but not quite so 
hard as felspar. Its color is usually a greenish- 
black. 
To distinguish between quartz and felspar is very 
often a beginner’s difficulty. The general reader may 
be so far interested as to desire to recognise felspar as 
a constituent of granite, or oven to separate that 
