366 A. KEITH OUTLINES OF APPALACHIAN STRUCTURE 



PROBABILITIES 



The probabilities in regard to the hypothesis of batholith intrusions 

 are as follows : An association of igneous intrusion with mountain-build- 

 ing is well known, and in the Appalachians enormous batholiths were 

 intruded at the time of Appalachian deformation. Unless these l^atho- 

 liths were under a pressure of their own, equal to or greater than the 

 pressure of the hard parts of the crust, they could not have entered the 

 hard crust; or if, perchance, they had entered before the pressure on the 

 crust began, they would have been promptly squeezed out by the harder 

 crust, like water from a sponge. The igneous rocks must, therefore, have 

 entered the crust after the deforming pressures began and thus must 

 have been under a pressure greater than that of the crust itself. It is 

 rational to conclude, therefore, not that there were two independent 

 pressures, one for the crust and one for the intrusives, of similar amount 

 and different origin, but rather that there was one force and the same 

 for both crust and intrusive. Force, therefore, lay in and began with the 

 intrusives ; otherwise they could not have entered the crust. There is no 

 probability that igneous intrusions would be rhythmic, for it is not 

 known that they depend on long-continuing causes like contraction 

 through loss of heat, or on any cosmic phenomena, such as change in 

 rate of revolution of the earth. In this respect the theory accords with 

 the lack of continuity or rhythm disj)layed in mountain-building. 



Most igneous intrusions show great force. This is particularly true 

 of batholiths, laccoliths, and sills, which plainly have forced apart enor- 

 mous rock masses or have lifted huge piles of overlying strata. The force 

 exerted is shown in detail in the shattering and rending of the rocks. 

 While many flows and dikes do not appear to have l)een under pressure, 

 yet the fact that they reached the surface at all and kept their channels 

 opened is evidence of considerable pressure in their depths. What we 

 now see of them are chiefly the portions which were released from 

 pressure. If it is true, as it seems to be, that oj^en spaces can not exist 

 a few miles deep within the crust, it is clear that any igneous rocks which 

 were deep-seated enough to be molten must have been under heavy 

 pressure or their channels would have remained closed. That this force 

 is quantitatively sufficient for mountain-building seems clear from the 

 fact that it is greater than the strength of the crust, for it has repeatedly 

 shattered the crust and moved the parts around. The argument from 

 probalnlity is, therefore, distinctly in favor of this hypothesis, inasmuch 

 as the great intrusive masses have had the requisite heat and pressure to 

 deform the crust greatly and have been associated with mountain- 

 building. 



