PHYSICAL PROPERTIES OF ROAD MATERIALS. 5 
Furthermore, the microscope reveals certain peculiarities which are 
not discernible with the naked eye. Thus, plagioclase feldspar 
crystals (PL VII, fig. 3), that appear perfectly homogeneous in the 
hand sample, are seen under the microscope to be made up usually 
of a great number of smaller individual crystals arranged in a charac- 
teristic manner, or again, certain constituents such as augite and 
hornblende (PL VII, figs. 4 and 5) are distinguished by microscopic 
lines of parting (cleavage) that intersect at definite angles quite 
distinct from each other. As these minerals all differ in chemical 
composition, their optical properties are also different; that is, when 
light passes through a transparent thin section of a rock under a 
microscope with properly adjusted Nicol prisms, certain minerals 
will appear in beautiful rainbow colors, while in others these inter- 
ference colors may be of a lower order and much fainter. Again not 
infrequently some components are encountered that show no optical 
reaction whatever and remain consequently dark through an entire 
revolution of the microscopic stage. These characteristic optical 
properties greatly facilitate the identification of the various rock- 
forming constituents. 
The chemical composition of a rock magma necessarily influences 
or controls the formation of minerals within the rock. Thus cer- 
tain magmas, rich in silica and potash, embracing so-called acid 
rocks, produce on cooling light-colored granite and rhyolite (PL I, 
figs. 1 and 2), composed chiefly of quartz, orthoclase, and mica (PL 
VII, figs. 1, 2, 8, and 9); while other magmas having less silica and 
more iron and lime give rise to darker basic or trap rocks, such as 
diorite, gabbro, diabase, and basalt (Pis. II and III), made up chiefly 
of hornblende, plagioclase, augite, and magnetite (PL VII, figs. 3, 4, 
5, and 11). The mineral components of sedimentary rocks are 
dependent in composition on the character of the materials from 
which their sediments were derived, as well as upon the nature of 
the matrix cementing them. Calcareous deposits, therefore (Table 
1, Nos. 15-16), derived chiefly from shell fragments (PL V, fig. 2), 
consist essentially of calcite and dolomite (PL VII, fig. 6), with vary- 
ing amounts of quartz, kaolin, and iron oxide as impurities, while 
sandstones (Table 1, Nos. 17-20), formed from siliceous rock mate- 
rials that have withstood the abrasive effects of transportation, are 
made up mainly of quartz and kaolinized feldspar fragments cemented 
together by calcite, limonite, or silicified clay (PL IV, fig. 1). Chert, 
or flint (PL IV, fig. 2), differs somewhat from a normal sandstone in 
that it is composed chiefly of very finely divided quartz frequently 
associated with opaline silica that originated from minute siliceous 
shells of marine animals through a process of chemical solution and 
deposition. The minerals present in metamorphic rocks are similar to 
those of igneous and sedimentary origin with the subsequent develop- 
