178 STRUCTURAL GEOLOGY. 



4. Continents and ocean-bottoms have not, as some 

 imagine, frequently changed places. On the contrary, 

 the places of continents have been indicated and their 

 outlines sketched out from the beginning, and their forms 

 have been gradually developed, though with many oscil- 

 lations, throughout all geological times. 



The origin of continents and ocean-bottoms is 

 very obscure, but it is probably in some way connected 

 with the unequal contraction and therefore deformation of 

 the spheroidal form of the earth, by slow cooling from a 

 former incandescent condition. In such an irregular or 

 deformed spheroid, of course, the water would collect in 

 the hollows, and the protuberances would become conti- 

 nents. The origin of mountains we discuss further on. 



Rocks. 



Definition of Rock. The term rock is used in popu- 

 lar language to designate any substance of stony hard- 

 ness. Not so in geology. Any substance constituting a 

 portion of the earth's crust, whether it be hard or soft, 

 is called a rode. No distinction based on hardness alone 

 is of any value. The same sandy bed may be found in 

 one place hard enough for building-stone, and in an- 

 other soft enough to be spaded. The same clay stratum 

 may sometimes be traced from a condition of slaty hard- 

 ness in one place to good brick-earth in another ; the same 

 bed of lime from marble into chalk, and the same volcanic 

 eruption from stony lava into a bed of volcanic ashes. 



Classes of Rocks. Rocks are divided, according to 

 their structure and origin, into two principal kinds, viz., 

 stratified and unstratified. Stratified rocks are more or 

 less consolidated sediments., and are therefore aqueous in 

 origin and earthy in structure. Unstratified rocks have 

 been more or less fused, and therefore are igneous in 

 origin and either crystalline or glassy in structure. 



