286 R. A. Daly — Mechanics of Igneous Intrusion. 



unlikely that total digestion would destroy blocks fallen from 

 the roof. They might, therefore, be looked for on the floors. 

 So far, the writer has discovered no evidence on the point in 

 any of the monographs. The reasons are not far to seek. Very 

 few floors of laccoliths are actually exposed. It is probable, 

 too, that in many instances an observer would have difficulty 

 in distinguishing blocks 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 unusu- 

 ally 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. The specific gravity of the Mt. Hillers 

 trachyte, for example, is at 20° C, 2'63. Using Delesse's ratio 

 of 4 per cent of net volumetric increase in the passing of 

 trachyte from rock to glass at the same temperature, and then 

 applying Barus's fusion curve, it follows that the trachyte 

 magma would have, at 1400° C. and one atmosphere of pressure, 

 a specific gravity of 2'34. From Reade's expansion coefficient 

 for sandstone we can calculate the specific gravity of the unal- 

 tered invaded sandstone at 1400° C, while conceived as still 

 solid and obeying the normal law of thermal expansion for 

 sandstone. The decline in specific gravity is from 2*15 to 

 2-06. The corresponding decline for the contact metamor- 

 phosed sandstone is from 2*48 to 2*38. The values show a 

 high probability that the average sandstone fragment would 

 float in the magma at least until it was altered nearly to the 

 maximum observed near Mt. Hillers. This conclusion raises 

 the suggestion whether density may not control the formation 

 of some laccoliths in a manner somewhat different from that 

 hypothecated by Gilbert;}; — a subject evidently remote from 

 the purpose of these pages. 



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 con- 

 ceivable 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 only negative evidence in 

 the test of the stoping 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. 



*Op. cit., p. 66. 



fThe Laccoliths of the Black Hills, U. S. Geol. Survey, 21st Ann. Eep., 

 Part III, 211 (1901). % Op. cit., p. 75. 



