368 A. KEITH OUTLINES OF APPALACHIAN STRUCTURE 



By analogy of similar cases, therefore, this theory of batholitic intru- 

 sion is supported in regard to the quantity of matter intruded, to the 

 geographic spaces inyolyed, and to the pressure necessary to accomplish 

 the yisible results. 



EXAMPLES 



The argument from example is necessarily more restricted than the 

 arguments from probability and analogy, but its tenor is the same. It 

 has been a matter of repeated comment that igneous actiyity and moun- 

 tain-building ^yere substantially coincident in many regions and times. 

 Both in the northern and southern portions of the Appalachian system 

 enormous masses of granite were intruded in great batholiths at sub- 

 stantially the time of the folding. In the central part of the Appa- 

 lachians (Penns3dyania and Maryland) the facts are largely concealed 

 by the Cretaceous deposits, but enough area remains to exhibit a few of 

 the batholiths of the same age as the others. That more are concealed 

 beneath the Cretaceous is probable from their eastward increase where 

 the system is more widely yisible. They were intruded under great 

 pressure, as is shown in thousands of places by the shattering and rend- 

 ing of the country rock. Thousands of dikes lead off from the batholiths 

 and hundreds of sills are associated with them. The width of the now 

 yisible portion of indiyidual batholiths (as great as 30 miles) is proof 

 of an enormous amount of separation of the country rocks — an amount 

 which is enormously increased by the distention due to sills and dikes. 

 It may be objected that a large part of this apparent distention is merely 

 substitution of the batholith for the country rock with remoyal of the 

 latter, or perhaps digestion of it. In answer to this, it may be said that 

 while there is eyidence of digestion it is limited to the margins of the 

 batholiths, whose general composition remains unaffected oyer enormous 

 areas. In detail the contacts of the intrusiye bodies are sharp, they cut 

 across layers of the country rock cleanly, and there is no eyidence of 

 anything in the form of general substitution. On the contrary, where 

 detailed mapping has been done, in Xorth Carolina, for instance, the 

 thrusting and wedging action of the scores of sills and tongues is par- 

 ticularly clear. A lifting and doming action is well shown in the Pisgah 

 region of North Carolina (Pisgah folio. United States Geological 

 Suryey). 



The ground plan of the batholiths in relation to that of the folding is 

 highly instructiye. The two main foci of intrusion are at the north and 

 south ends of the Appalachians. Parallel to these intruded areas, and 

 just northwest of them, is seen the greatest deyelopment of thrust faults. 



