29± R. A. Daly — Mechanics of Igneous Intrusion. 



the voluminous flows at master fissures, it is clear that the 

 corresponding magmas had relatively easy access to the surface 

 and had not to work their own way through the earth's crust. 

 The plateau lavas therefore merit particular notice in the search 

 for the general earth-magma or magmas. It is doubtful that 

 enough emphasis has been placed on the volume, relative abund- 

 ance and mode of geological occurrence characterizing the dif- 

 ferent eruptive types, in the published discussions on the origin 

 of igneons rocks. Those elements must always be of prime 

 importance in the solution of the problem of assimilation. 



For different reasons excepting that deriyed from the enor- 

 mously greater abundance of basaltic lavas on the earth, Dutton 

 came to this same conclusion as to the nature of the "primor- 

 dial matter." He has rightly dwelt on the fact that basalt is a 

 " synthetic or comprehensive type of rock." His theory of 

 the derivation of other igneous rocks by simple fusion of sedi- 

 mentary formations derived in their turn, by weathering, from 

 the " primordial matter," takes insufficient account of the facts 

 of differentiation learned since 1880. Yet his theory has a 

 suggestive relation to the one proposed in these pages.* 



It is not essential to our present purpose to decide on the 

 question whether the acid-alkaline magmas of the continents 

 represent the more or less altered primal material segregated 

 on the original crust of the earth, or are the product of the 

 secular alteration of continental sediments derived from the 

 synthetic basaltic magma — if such decision be really possible. 

 The main facts of igneous rock distribution and the a priori 

 conception that alkaline magma must float on the ferromag- 

 nesian magma, lend themselves to the belief that a gabbroitic 

 magma underlies the surface of the whole earth at a depth not 

 too great to prevent its energetic eruption. If, during such 

 eruption, overhead stoping and abyssal assimilation accom- 

 panied by differentiation, occur, the gabbroitic magma will be 

 modified and a normal world sequence from basic to acid be 

 established. In spite of the numerous exceptions to this order 

 in nature, a fair judgment on the case must reach the conclu- 

 sion that such a law governs intrusive bodies at least. We have 

 already seen reasons for believing that the order may, on the 

 same hypothesis, be abnormally reversed for plutonic masses, 

 and should be reversed for the greater volcanic rock-bodies. 

 Granite, on account of its superior stoping power and low 

 density, compared with the basic plutonic rocks, should be the 

 more common among those irruptives exposed by denudation 

 at the earth's surface. 



*Rep. High Plateaus of Utah, p. 125 ff., Washington, 1880. 



