178 FIRST YEAR COURSE IN GENERAL SCIENCE 



In the course of time, many of the rocks were changed into 

 crystalline rocks. From that far-away age of the world 

 even up to the present time, rocks have been, and still are, 

 in the process of formation. 



Rocks are divided, according to the way they were made, 

 into three classes: igneous, sedimentary, and metamorphic. 



FIG. 89. A GRANITE QUARRY 



Observe the irregular surfaces of the rock exposed, unlike those of Figs. 

 90 and 91. When this igneous rock contracted as it solidified, cracks called 

 joint planes were made. 1. What is their general direction with regard to 

 the earth's surface ? 2. Are they a help or a hindrance to quarrying ? 



202. Granite and other Igneous Rock. Igneous rocks 

 are those which originally rose in a melted condition from 

 within the earth, and afterwards cooled either near the 

 surface or often very far below it. The microscope shows 

 these rocks to be made of crystalline grains of different 

 sizes, neatly fitted together. 



