178 NATURE STUDY 



the disintegratioa of the mica aud consequent breaking 

 of the rock itself. 



Mica: 



The commonest of all the micas is white mica, or 

 rauscovite ; the next, black mica, or biotite. Both of 

 these are found in fiat, six-sided forms, made up of very 

 thin, transparent, elastic layers. Both are quite soft, 

 being easily scratched with the finger nail. "VMiite mica 

 may be white, gray, or, less frequently, brown or yellow. 

 Black mica is deep black to green. Moreover, it is 

 almost opaque when viewed in one direction, and trans- 

 parent and of a different color, when viewed in another 

 direction. Mica is used in stove doors, for window 

 glass in Siberia, for covering compass boxes, for frost 

 work at Christmas time, and sometimes, like graphite, 

 for lubricating machinery. 



Sand: 



The next most evident thing in soil is usually sand. 

 The grains are easily picked out, and will be found to 

 vary in color, some being transparent; others, milky 

 white ; others, still, yellow. They are all grains of quartz, 

 hard, therefore, and gritty. They have rubbed against 

 each other until they have lost their sharp angles. Sand 

 is used in the manufacture of glass, plaster, mortar, and 

 for scouring and moulding. 



Clay: 



The clay in soil may be easily recognized by its char- 

 acteristic odor, particularly when wet. It is, in most 

 instances, derived from disintegrated feldspar. The pres- 

 ence of too much clay in the soil makes it stiff and 



