40 ELEMENTARY GEOLOGY 
so liable to decay. Besides being present in the rocks, 
it is always found in small percentage dissolved in the 
waters of lakes, oceans, springs, and rivers. 
The Feldspar Group. — Under the name feldspar, is | 
included a great variety of substances which resem- 
ble one another in some respects, but present enough 
points of difference to enable the mineralogist to distin- 
guish them. They are all silicates of alumina (SiO, 
and Al,O;) with some other elements, such as magne- 
slum, potassium, calcium, etc. Each species differs 
slightly from the others in chemical composition and 
mineral form. ‘The two most common divisions are 
orthoclase and plagioclase, which in the ordinary rock 
specimen, cannot be told apart by the eye alone. 
Like quartz, feldspar is variable in color, and light in ~ 
weight. It is only a little less hard than this mineral; 
but, while quartz always breaks with irregular face, 
feldspar, when broken, is found to split in some direc- 
tions with perfectly smooth faces. This is cleavage, 
and the planes of breakage are known as cleavage 
planes (Fig. 4). The cleavage of feldspar, which is 
always present, is a part of its habit of ecrystalliza- 
tion; for feldspar is always crystalline, though good 
crystals are not common. 
To the geologist there are two very important ways 
in which feldspar differs from quartz: (1) It is not 
even so soluble as the nearly insoluble quartz; but 
