168 SSM GOV ORS MOV SIKGUOVIINIES 
feldspar takes place very slowly, but owing to the fact that it 
often occurs in granite and other rock of great age, it is fre- 
quently in an advanced stage of alteration. The alteration 
products of feldspar are objectionable only in so far as they 
yield more readily to disintegration. 
Mica is also a very common mineral, occurring most abun- 
dantly in the metamorphic rocks. The ready cleavage by which 
the mineral splits into thin plates, provides an easy passage for , 
water, by which disintegration proceeds more rapidly than in 
the associated minerals. Mica is undesirable in proportion to 
the size of the individuals. If present in small isolated flakes, as 
it ordinarily occurs in sandstone, it is scarcely less durable than 
quartz and feldspar, but if the individuals are large or the flakes 
clustered together, disintegration will proceed more rapidly. 
Decomposition through chemical agencies goes on very slowly. 
Calcite is almost as common as quartz, although far less per- 
manent at the surface of the earth. It possesses three prominent 
cleavage directions, on account of which it disintegrates quite 
readily. The hardness, and probably the strength and elasticity, 
are all less than in quartz. It is quite easily soluble in carbon- 
ated waters and is readily acted upon by cold, dilute hydro- 
chloric acid. 
Dolomite differs from calcite mainly in its somewhat greater 
hardness, and the greater difficulty with which it dissolves in 
cold dilute hydrochloric acid. Its cleavage, hardness, strength, 
and elasticity are such that it disintegrates almost as readily 
as calcite, although it is taken into solution somewhat more 
slowly. 
Kaolin is an important constituent of slate, being however, 
mainly of secondary origin. It is one of the softer minerals, has 
a perfect cleavage, and readily disintegrates. It is not acted 
upon chemically except under the most favorable conditions. 
Pyroxene is one of the less important building-stone minerals. 
It cleaves perfectly in two directions, and breaks down slowly 
through mechanical abrasion. It gradually decomposes in the 
quarry when in the presence of water. 
