760 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



have been ' entirely transformed ' into porous glass* Von John has described 

 other examples of the same transformation. In chapter X. the present writer 

 has correlated a considerable number of instances where the gravitative strati- 

 fication has certainly been produced in thick intrusive sheets. 



A number of observers have come to the conclusion that the very act of 

 the assimilation of acid material by basalt predisposes the magma to magmatic 

 splitting. The fullest statement of this view is given by Loewinson-Lessing, in 

 his remarkable ' Studien iiber die Eruptivgesteine.'f There appears to be, as it 

 were, a steady ' antagonism ' between the f erromagnesian and acid-alkaline 

 elements in magmas. This primordial tendency toward immiscibility may well 

 explain the dominant acidity and alkalinity of the pre-Cambrian terranes in 

 every continent. From the earliest times the granito-rhyolite magma has tended 

 to separate from the basaltic wherever the viscosity has been sufficiently low 

 for such splitting. For similar reasons it appears that the syntectic magma 

 of post-Archean batholiths only reaches a stable condition when it assumes the 

 ancient relation. In the average case the fluidity has been high enough for the 

 splitting. In some cases, however, it was so low that the undifferentiated 

 syntectic has crystallized as diorite and allied rocks. 



When the syntectic has differentiated, the process must be primarily con- 

 trolled by density, so that the acid, generally granitic, product rises to the top 

 of the chamber. There it may become locally further differentiated through 

 fractional crystallization or other relatively subordinate process. 



Without discussing the causes of differentiation in more detail it suffices 

 tc point out, in summary, that magmatic stoping involves the placing of gravity 

 at the head of the list of forces which produce the actual diversity among igneous 

 rocks. In this the stoping hypothesis is believed to match the facts observed 

 in experimental, industrial and geological studies of silicate melts. 



Origin of Granite; the Petrogenic Cycle.— The stoping hypothesis involves 

 a more or less definite corollary relating to the genesis of granite as the staple 

 visible material of post-Archean batholiths. Erosion has nowhere penetrated 

 more than a few thousand feet in any of these batholiths. Considering the 

 scale of operations, it follows that practically all post-Archean batholithic rock 

 is of secondary origin. The field relations show that the granite often replaces 

 much geosynclinal sediment. Thick as many geosynclinal prisms are, however, 

 it seems clear that another large, probably the larger, part of the replaced rock 

 is the pre-Cambrian crystalline terrane (averaging granitoid gneiss in chemi- 

 cal composition) which underlies geosynclinal areas, as it apparently underlies 

 all the continental areas. The similarity of granites throughout the world may, 

 indeed, be partly explained by the uniformity of the earth's primordial, acid 

 shell and by the relative uniformity in average chemical composition of the 

 greater geosynclinal prisms. 



A speculation as to the acid shell is noted on pages 702 to 705. It views the 

 shell as possibly an anchi-eutectic derivative of an intermediate (andesitic) 



*Les Enclaves des Roches Volcaniques, p. 563-5; Macon, 1892. 



t Compte Rendu, Congres ge"ol. internat, Vile session, St. Petersburg, 1899, p. 375. 



