8 BULLETIN 348, U, S. DEPARTMENT OF AGRICULTUEE. 
Common mica is readily recognized in the hand sample by a thin 
plate-like crystal form, frequently with hexagonal outline (PL VII, 
figs. 8 and 9) and more especially by a perfect basal cleavage which 
causes the crystals to part readily into thin elastic flakes. The foli- 
ation characterizing many types of metamorphic rocks is caused 
largely by the parallel arrangement of mica laminae, which is also 
largely responsible for the poor wearing properties of this material. 
Magnetite and Garnet (PI. VII, figs. 11 and 12) are found as essen- 
tial constituents in but two types of road material (peridotite and 
eclogite). They occur in well-defined crystal form without cleavage 
and are extremely heavy (specific gravity 5.18-3.15), and hard 
(5.5-7.5 1 ) and in some cases materially increase the wearing properties 
of the rock. (See Table 7, No. 13.) " 
Besides the above-mentioned primary minerals others are found 
widely distributed in various kinds of road materials, but generally 
in such small quantities as not to affect appreciably the physical char- 
acter of the rocks. Among these accessory constituents may be men- 
tioned hypersthene, olivine, titanite, apatite, zircon, and pyrite. 
SECONDARY MINERALS. 
Under secondary minerals will be considered only the more 
important products of rock decay brought about mainly by the 
chemical action of water and carbonic acid on primary rock con- 
stituents. 
Calcite and Dolomite (PL VII, fig. 6) are the chief constituents of 
limestones and dolomites and so far as they have formed upon the 
first consolidation of this material might consistently be classed as 
primary. In all other rock types these carbonates are distinctly 
of secondary origin and formed by the chemical combination of 
carbonic acid with lime and magnesia derived either directly from 
the disintegration of primary constituents or introduced later from 
extraneous sources. These minerals have approximately the same 
physical properties and can not be separated microscopically, but 
may be readily distinguished by their different behavior toward 
cold dilute acids; calcite being readily dissolved with vigorous 
ebullition of carbonic acid gas, while dolomite is very slowly acted 
on with barely perceptible effervescence. Calcite and dolomite are 
soft minerals (hardness 3-3.5 x ) ; of moderate specific gravity (2.6- 
2.9), and occur always in a crystalline condition. The crystals are 
characterized by perfect cleavage in three directions corresponding 
with the planes of a rhombohedron and for this reason calcareous 
rocks as a rule break down readily into fine crystalline powders. 
Kaolin is a very common product of rock decay, especially in 
temperate climates, and is derived to a large extent from orthoclase 
i Mohs scale. 
