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



This reasoning is deductive but it can in some measure be 

 checked by actual observations. Lacroix describes blocks of 

 gneiss up to a cubic meter in size, which have been immersed 

 in molten basalt. By the heat of the lava the blocks have 

 been " entirely transformed " into porous glass.* Yon John 

 lias described other examples of the same transformation. f 

 The present writer has correlated a considerable number of 

 instances where the gravitative stratification 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 predis- 

 poses the magma to magmatic splitting. The fullest statement 

 of this view is given by Loewinson-Lessing. in his remarkable 

 " Studien iiber die Eruptivgesteine."§ There appears to be, 

 as it were, a steady *■ antagonism " between the ferromagnesian 

 and acid-alkaline elements in magmas. This primordial tend- 

 ency 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 controlled 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 to point out, in summary, that magmatic stop- 

 ing 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 



*Les Enclaves cles Koches Yolcaniques, p. 563-5; Macon, 1892. 



+ Op. cit., p. 141. 



% This Journal, xx, p. 185, 1905: also Festchrift zum siebzigsten Geburts- 

 tage von H. Rosenbusch, p. 203, Stuttgart, 1906. 



§ Coinptes Rendus, Congres geol. internat, VII e session, St. Petersburg, 

 p. 875, 1899. 



