170 GENERAL FORM AND STRUCTURE OF THE EARTH. 



2. In each continent the greatest range of mountains faces the 

 greatest ocean. Thus in America the greatest range is on the west, 

 facing the Pacific ; while in Africa the greatest range is on the east, 

 facing the Indian Ocean. In Asia the Himalayas face the Indian 

 Ocean, while the Altai face the Polar Sea. In Australia the greatest 

 range is to the east, facing the Pacific. 



3. The greatest ranges have been subjected to the greatest and 

 most complex foldings of the strata, and are the seats of the greatest 

 metamorphism (p. 221) and the greatest volcanic activity. 



4. The outlines of the present continents have been sketched in the 

 earliest geological times, and have been gradually developed and per- 

 fected in the course of the history of the earth. In the case of the 

 North American Continent this will be shown in Part III. 



The cause of some of these laws will be discussed under the head 

 of Mountain-Chains. 



Rocks. 



In geology the term rock is used to signify any material consti- 

 tuting a portion of the earth, whether hard or soft. Thus, a bed of 

 sand or clay is no less a rock than the hardest granite. In fact, it is 

 impossible to draw any scientific distinction between materials founded 

 upon hardness alone. The same mass of limestone may be soft chalk 

 in one part and hard marble in another : the same bed of clay may be 

 hard slate in one part and good brick-earth in another ; the same bed 

 of sandstone may be hard gritstone in one part and soft enough to be 

 spaded in another. The same volcanic material may be stony, glassy, 

 scoriaceous, or loose sand or ashes. 



Classes of Rocks. — All rocks are divided into two great classes, viz., 

 stratified rocks and unstratified rocks. Stratified rocks are more or 

 less consolidated sediments, and are usually, therefore, more or less 

 earthy in structure and of aqueous origin. Unstratified rocks have 

 been more or less completely fused, and therefore are crystalline in 

 structure and of igneous origin. 



CHAPTER II. 



STRATIFIED OR SEDIMENTARY ROCKS. 



Section 1. — Structure and Position. 



Stratification. — Stratified rocks are characterized by the fact that 

 they are separated by parallel division-planes into larger sheet-like 

 masses called strata, and these into smaller layers or beds, and these 



