708 A TREATISE ON METAMORPHISM. 



MANNER OF INTBVSION. 



The manner of intrusion in the zone of anamorphism differs in many 

 respects from that in the zone of katamorphism. In the latter zone it has 

 been explained that the igneous rocks mainly follow fractures. In the 

 lower zone no large fractures can be supposed to exist continuously. 

 In some way the magma must make its way through rocks which normally 

 have no large opening's. 



The progress of the igneous rocks is therefore a matter of far greater 

 difficulty. In consequence of earth movements the material may make its 

 way (1) en masse, without breaking across the structures of the intruded 

 rocks to a great extent; (2) by breaking across the structures of the 

 intruded rocks and by following along- planes of weakness such as cleavage, 

 bedding, and contacts; and (3) by fusion and absorption of the intruded 

 material. 



(1) It is probable, indeed certain, that all three of the above processes 

 occur to some extent, but it is believed that the movements en masse, 

 without extensive breaking across the intruded rocks, are of great conse- 

 quence. As evidence of this may be cited the fact that in general, adjacent 

 to the great batholitbs, the injected rocks have peripheral, structures, as if 

 they had been driven aside en masse by the earth stresses in connection 

 with epeiorogenic and orogenic movements. It is believed that the magmas 

 transmit the thrusts they receive substantially under the laws of hydro- 

 statics, and consequently make their way by raising' up and pushing aside 

 the material previously occupying the space, without necessarily breaking- 

 through it on a large scale. Under such circumstances, if the intruded 

 rock be sedimentary, the bedding of the sediment is peripheral to that of 

 the great intrusive masses. Materials of other kinds are similarly arranged. 

 The intrusive acts in reference to the rocks as would a hemispherical or 

 lenticular mass raised against a piece of flexible but tightly stretched 

 leather. The leather roughly adjusts itself to the shape of the hemisphere 

 or lenticule. 



Also, as will be seen, high pressure, temperature, and water result in the 

 recrystallization of the rocks adjacent to intrusive masses, and during this 

 process they are shortened in a direction normal to the intrusion and 

 sometimes lengthened in a direction peripheral to it. Thus room is made 

 for the entering mass. Often the result, so far as the relations of the 



