PHYSICAL PROPERTIES OF ROAD MATERIALS. 5 



Furthermore, the microscope reveals certain peculiarities which are 

 not discernible with the naked eye. Thus, plagioclase feldspar 

 crystals (PI. VII, fig. 3), that appear perfectly homogeneous in the 

 hand sample, are seen under the microscope to bo made up usually 

 of a great number of smaller individual crystals arranged in a charac- 

 teristic manner, or again, certain constituents such as augito and 

 hornblende. (PI. VII, figs. 4 and 5) are distinguished by microscopic 

 lint^s 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 bo 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 (PI. 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 

 deposi tion. The minerals present in metamorphic rocks are similar to 

 those of igneous and sedimentary origin with the subsequent develop- 



