748 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



torn out of the floor from those sunk thither from the roof. Gilbert,* Jaggar,f 

 and others describe fragments at levels above the floor, but do not directly raise 

 the question as to how they were held suspended within the magma. In the 

 laccoliths of the Henry mountains, the unusually low densities of the invaded 

 sandstones and shales are such as to warrant the belief that fragments of these 

 rocks really floated in the magma. 



Jaggar has described large blocks of Cambrian strata as immersed in the 

 laccolithic porphyries of the Black Hills and explains them as due to ' excessive 

 doming.' Yet it is conceivable that they may owe their present positions to 

 high magmatic viscosity, the magma freezing as they were in the act of slowly 

 floating upwards from the floor or sinking from the roof of the laccolith. 



So far, then, laccoliths have given chiefly negative evidence in the test of the 

 sloping hypothesis for plutonic magmas, and, perhaps in the nature of the case, 

 they can never be of great value in determining the truth of the hypothesis.:}: 



Problem of the Cover. — The stoping hypothesis presents an obvious prin- 

 cipal difficulty; it refers to the apparent danger of the foundering of the roofs 

 covering the larger batholiths. Under plutonic conditions (at depths of from 

 one to five or six miles) the average molten granite would have a specific gravity 

 no higher than 2-40. The average rock of its roof has a specific gravity of 

 about 2-70. If, then, through orogenic movement, a large mass of the roof -rock 

 became once wholly immersed in the granite, it would not only founder itself 

 but through subsequent buckling the whole roof might collapse and founder 

 in sections. Doubtless such a catastrophe has seldom happened in the case of 

 any Paleozoic or later batholithic intrusion. This difficulty has been emphasized 

 by Barrell, who justly gave it a prominent place in his monograph. § 



The present writer cannot claim to have solved this problem, but he does 

 not find it to form a fatal objection to the hypothesis. In the first place, it 

 seems clear that all the other hypotheses of granitic intrusion are facing the 

 same dilemma. All of them expressly or tacitly postulate some degree of fluidity 

 in each granitic mass as it either replaces or displaces its country-rocks. We 

 have seen that, though the viscosity of such a magma may be several hundred 

 times that of water, the roof-sections, once immersed, must sink in the magma. 

 All petrologists who believe in magmatic or other differentiation as operative 

 in batholiths must face the common difficulty. 



Secondly, the writer has shown reasons for believing that the earth's crust 

 at present rests on a continuous couche of basaltic (gabbroid) magma, either 

 quite fluid or ready to become fluid when injected into the crust. If the average 

 specific gravity of the crust is 2-75 (a probable value), it would as a whole be 

 quite able to float on the basaltic couche, which, under the great pressure, would 

 probably have a specific gravity over 2 -SO. Imperfect as the numerical data 



* G. K. Gilbert, Geology of the Henry Mountains, 1877, p. 66. 

 tT. A. Jaggar, U.S. Geol. Survey, 21st Annual Beport, Part 3, 1901, p. 211. 

 t Do the "muscovado" blocks on the floor of the Duluth gabbro "laccolith" of 

 Minnesota in part represent sunken fragments of its roof? 



§ J. Barrell, Prof. Paper, No. 57, U.S. Geol. Survey, 1907, p. 172. 



