286 UNSTRATIFIED ROCKS 



component crystals usually increases from the circumference to 

 the centre of the mass. 



Bathyliths are huge masses of igneous rock, which may be 

 scores or hundreds of miles in extent, and are of entirely irregular 

 shape. Like bosses, they are exposed to view only when de- 

 nudation has cut the surface down to the level at which they 

 were formed. It is difficult to understand how such vast quanti- 

 ties of material could have been forced upward, except by melt- 

 ing their way, at least partly, through the overlying rocks. 



From this brief description it will be apparent that the various 

 forms of igneous rock which present themselves to our study are 

 the outcome of the interaction of several factors, such as the 

 ascensive pressure, the resistance to be overcome, and the fluidity 

 or stiffness of the molten magma. Such masses have played a 

 very important role in the modification of the earth's surface, 

 both by the displacement of previously existing rocks and by the 

 addition of new and different material. 



