8 BULLETIN 348, U. S. DEPARTMENT OF AGRICULTURE. 



Common mica is readily recognized in the hand sample by a thin 

 plate-like crystal form, frequently with hexagonal outline (PI. 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- 

 stitutents. 



Calcite and Dolomite (PI. VII, fig. 6) are the chief constitutents 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 constitutents 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 *), of moderate specific gravity (2.6- 

 2.9), and occur always in a crystalline condition. Th,e crj^stals 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. 



