H. A. Daly — Mechanics of Igneous Intrusion. 45 



to the genesis of granite as the staple visible material of post- 

 Archean batholiths. Erosion has nowhere penetrated more 

 than a few thousand meters in any of these batholiths. Con- 

 sidering 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 geo- 

 synclinal sediment. Thick as many geosynclinal prisms are, 

 however, it seems clear that another large, perhaps the larger, 

 part of the replaced rock may be pre- Cambrian crystalline 

 materials (averaging granitoid gneiss in chemical composition) 

 which underlie geosynclinal areas, as they apparently underlie 

 all the contiDental areas. The similarity of granites throughout 

 the world may, indeed, be 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. 

 Where sediments only are assimilated, the secondary granite 

 may be of abnormal composition ; this is the case with the 

 granite of the Moyie Sill."* 



The longer an abyssaily injected and assimilating body holds 

 its fluidity, the more perfect should be the gravitative differ- 

 entiation. During this active stage lateral fissures or laccolithic 

 spaces may be tilled with offshoots of the slowly changing magma. 

 In general these satellitic injections should succeed each other 

 in the order of increasing acidity. In a fully represented 

 petrogenic cycle at a batholithic area, then, the oldest intru- 

 sion should be a rock of gabbroid (basaltic) composition and 

 the youngest an acid granite (chemically a rhyolite or quartz 

 porphyry). Between these two an indefinite number of inter- 

 mediate rock-types varying according to their degree and kind 

 of differentiation from the syntectic — itself continuously vary- 

 ing in composition — might be represented in dikes or other 

 satellitic forms. This further deduction from our hypothesis 

 seems to be fairly matched by the observed order of igneous 

 intrusions about the world's batholiths.f 



Again, successive batholithic intrusions in the same area 

 should show the same law of increasing acidity with decreas- 

 ing age. If, for example, a crystallized granodiorite batholith 

 be itself attacked by a later abyssal intrusive and in large part 

 stoped away and remelted, the secondary magma collecting at 

 the roof of the later batholith should be more acid than 

 granodiorite. This would be expected because the mere act 

 of remelting entails further gravitative differentiation. Each 

 time that a silicate mass passes through the optimum tempera- 

 ture for magmatic splitting — probably an interval of one or 



* This Journal, xx, p. 196, 1905. 

 f See first intrusion paper, p. 292. 



