292 B. A. Daly — Mechanics of Igneous Intrusion. 



orations* are accordant with the hypothesis. An interesting 

 statement of it is given by "Walker, f In view of the agitation 

 produced by the powerful currents set up under the special 

 conditions inferred by the stoping and sinking of blocks, it is 

 also probable that gravitation would be capable of stratifying 

 the material directly dissolved by a magma from the walls of 

 its chamber. Secondary and less important causes may explain 

 the existence of complementary dikes, basic borders and 

 segregations. 



The eruptive sequence. — The particular kind of igneous rock 

 erupted from the magma basin will evidently depend on the 

 level from which the magma is drawn. In general, surface, 

 hypabyssal, and even plutonic bodies may be expected to 

 originate in the upper portions of the basins. If the original 

 magma be more acid than the invaded formation, the former 

 would probably, in its upper part, be modified by abyssal 

 assimilation in but subordinate degree, and the sequence of the 

 eruptions would normally be from acid to basic. If, on the 

 other hand, the original magma be more basic than the invaded 

 formation, that magma would be specially altered in the upper 

 part of the basin and a long continued series of eruptions would 

 show the order of basic to acid. Two different petrogenic 

 cycles are thus possible. By thorough solidification of the 

 upper layers of the magma (accompanied by secular denuda- 

 tion at the earth's surface), and renewed eruptivity, the petro- 

 genic cycle for an area might be repeated as a whole or in part. 



In deciding as to which of the two cycles should, by our 

 hypothesis, be most commonly represented in nature, it is clear 

 that each eruptive field and petrographic province should be 

 studied by itself and the results correlated for the world. The 

 impossibility of such complete correlation at the present time 

 need not discourage the attempt to test our hypothesis by such 

 generalizations as may now be made. Can we arrive at a 

 decision as to what is the average invaded formation and what 

 the average primary magma of the earth % 



The first question may be answered with a high degree of 

 probability for about one-third of the earth's surface, namely, 

 the continents and the immediately adjoining belts of the sea- 

 floor. In those areas, the greatest volume of rock above the 

 average isogeotherm of rock-fusion is doubtless made up of the 

 acid crystalline schists. The composition of the suboceanic 

 crust generally is almost entirely a matter of speculation. 

 The second question is beginning to assume a most prominent 

 place in petrogenic theories. On the continents, the staple 

 plutonic rock is the acid granite ; the staple volcanic eruptive, 



*Mikros. Physiographic, etc., ii, 552 (1896). 



fThis Journal, vi, 410 (1898) ; cf. Vogt. Zeit. fur prak. Geol., 1893, p. 279. 



