70 OUR PHYSICAL WORLD 



heavier than rhyolite pumice. It was thrown out originally as 

 coarse ash from the throat of the volcano and later solidified. 

 If the ash were thrown out in coarse fragments and these 

 were later cemented together, the rock is known as a basaltic 

 breccia. 



The trachytes, andesites, and basalts are so fine-grained 

 that it is difficult to distinguish them in the field, so for practical 

 purposes they are distinguished as felsites and basalts. If such 

 a fine-grained rock is very dark, grayish, greenish, purplish, or 

 black, the rock is called a basalt. If, however, the color is light, 

 medium gray, pink, red or even dark red, yellow, brown, or 

 light green, it is termed felsite. 



Finally, the peridotites are very heavy rocks in which there 

 is very little or no feldspar, the dominant minerals being pyroxene 

 and hornblende together with considerable iron ore. 



These igneous rocks would be largely wanting in the regions 

 covered by the sedimentary deposits, such, for instance, as the 

 states of the North and Central United States, were it not for 

 the fact that the great glacier which at one time covered this 

 region brought down with it great quantities of these rocks 

 imbedded in its mass or riding on its surface from the regions 

 occupied by the older rocks in Canada or the northern portions 

 of the states bordering the Great Lakes. When the glacier 

 finally melted and retreated, these rocks were deposited in the 

 soil as bowlders, so that the student even in regions where 

 the bed rock is sedimentary rock may find many samples of the 

 igneous rocks described above by collecting samples of these 

 bowlders. 



The chief sedimentary rocks are limestone, sandstone, con- 

 glomerate, breccia, shale, slate. When shells of such animals as 

 clams, oysters, snails, are worn to sediment by wave action, or 

 when the hard parts of coral are similarly disintegrated and the 

 sediment deposited in the quieter depths of the sea, then later by 

 the pressure of overlying layers and the heat of the earth is 

 changed to rock, the result is limestone. One marvels that the 



