50 C. R. VAN HISE 



What is true of the intrusive rocks of America is true of 

 other regions of the globe. I have selected America as an 

 illustrative continent, because I know the facts of the field 

 better there than elsewhere. 



Any terms which one can use must fail to convey an ade- 

 quate idea of the stupendous quantities of magma which have 

 been introduced into the outer shell of the earth, or poured out 

 upon its surface. It is clearly impossible to make even an 

 approximate quantitative guess of the amount of igneous mate- 

 rials which have thus been intruded and extruded during geo- 

 logical times. Its quantity is certainly to be measured in tens 

 of millions of cubic miles, rather than in smaller units. 



Now in this transfer of earth material two things have hap- 

 pened. In so far as it has been taken from the nucleus, it has 

 lessened its bulk. By the amount the nucleus has been lessened, 

 the bulk of the shell has been increased. 



Of this great mass which has thus migrated from the nucleus 

 to the shell, a large proportion has stopped before reaching the 

 surface. This is only possible by extension of the shell either 

 vertically or laterally, or both. The forms of intrusives clearly 

 show that both have locally occurred. Sills and laccoliths have 

 mainly found a place to occupy by vertical extension of the 

 shell, although to some extent lateral extension of the intruded 

 layers (see p. 15) is also produced by them. It is equally 

 clear that volcanic necks, dikes, and batholiths have largely found 

 space by local lateral extension, although it is not doubted that 

 the intrusion of such forms is also accompanied by vertical 

 extension, and in the case of batholiths an important amount. 

 Necks, dikes, and batholiths have formed in cracks and crevices, 

 and wedged the walls apart, thus locally extending the crust, 

 and giving surface which may be used in lateral mashing or cor- 

 rugations elsewhere. In many cases the mashing and corruga- 

 tion, and consequent thickening and vertical extension, are 

 immediately adjacent to the intrusives. This is most marked 

 in the case of the great batholiths. Adjacent to such enormous 

 masses as the batholiths which are found in the Black Hills, in 



