during the Cooling of Intrusive Masses of Granite. 235 



cisterns*, and are, in fact, reservoirs fed from a throat below. 

 In other cases it may be possible that they are connected 

 with the central igneous magma of the earth in a larger way, 

 as tongues or protrusions of it. When this is the case it is 

 difficult to realize the enormous amount of heat-energy 

 brought up from below to be expended on the overlying 

 sediments in the way of mountain-building. Whether the 

 granite intrusion be fed by a throat, or whether it be a 

 tongue of molten matter, as it cools and congeals within 

 the bowels of the earth (which it must do at an extremely 

 slow rate) the uprising stream will introduce compensating 

 matter in the same manner as does the plug in casting. 

 The only difference will be that the one runs in by 

 gravitation, the other is forced up by pressure from below. 

 But while the matter is rising it is also eating into the 

 surrounding sediments and incorporating them into its own 

 bod}'. And it is well here to remember that granite is of a 

 higher specific gravity than most sedimentary rocks, in the 

 proportion of about 2*90 to 2*30, or, in other words, a ton 

 of granite contains from 11*8 to 12*8 cubic feet, while a ton 

 of sandstone contains from 14*3 to 17*3 cubic feet. Therefore, 

 if we for the sake of illustration assume a sandstone to 

 consist of the same elements as a granite, it will, on incorpora- 

 tion with the molten mass, actually decrease in volume. Of 

 course the amount of decrease would vary with the com- 

 position of the sandstone. Again, if the surrounding rock 

 were marl it would decrease in a greater ratio. 



There are thus two forces at work — one the absorption of 

 the surrounding rock and its decrease in volume, the other 

 the introduction of new matter, the total effect tending 

 towards disruption by increasing bulk. This effect may be 

 seen in any good granite contact. There is generally a 

 laminated gneissic band between the granite and the enclos- 

 ing rock, and often they are so welded together that junction 

 specimens showing both structures can be taken and cut for 

 the microscope. It is not improbable that the fragments 

 of gne^'ssic rock found in gTanite may be, so to speak, broken 

 off from the walls of the intrusion. 



Signs of lateral displacement of the surrounding rock 

 to a considerable extent are also sometimes seen. But what 

 is it that prevents the disruption and fissuring of the granite 

 by contraction after solidificat'on takes place ? 



I answer gravitation, for one thing ; for so long as the rock 

 will yield as a paste, the mass above, wjiich in most cases is 

 supposed to have been very considerable, will keep it com- 

 * Geology of the Henry Mountains. 



