R. A. Daly — Mechanics of Igneous Intrusion. 43 



The gabbros of Paleozoic or later age represent bodies either 

 too small or of too low temperature to carry on extensive stop- 

 ing before their magmas became rigid. Diorite stocks and 

 batholiths, according to the hypothesis, represent undifferen- 

 tiated or but partially differentiated syntectic magma — of com- 

 position intermediate between rhyolite or granite and basalt. 

 The average chemical analyses of the world's basalt, granite 

 and diorite have been calculated by the writer from Osann's 

 compilation.* It has been found that the diorite analysis is, 

 oxide for oxide, almost the exact mean between the other two 

 analyses. 



These various considerations incline the writer to the view 

 that the existence of a few large basic intrusions, cutting acid 

 rocks, is not necessarily a fact fatal to the stoping. hypothesis. 

 Each of the cases needs special study, for they may shed much 

 light on the difficult plutonic problem. 



Differentiation of the syntectic magma. — In order to trace 

 further the history of the engulfed xenoliths several principal 

 conditions must be recognized. If the invading magma is 

 superheated, so as to have the temperature of 1300° C, a 

 block of heavy gneiss (sp. gr. at 20° C, 2*85) will speedily be 

 heated to and above its own melting-point. While some of it 

 is dissolved, much of it is converted into a molten globule of 

 essentially pure gneiss. From Table II we see that the 

 specific gravity of the globule would be about 2'40, while that 

 of the 'surrounding primary magma would average about 2'72. 

 This difference of density means that the globule must rise 

 through the primary magma with a speed even greater than 

 that with which the solid rock (specific gravity about 2*75) 

 formerly sank.f As it rises the globule would wholly or 

 partly mix with the primary magma. If wholly mixed the 

 primary magma rapidly becomes a syntectic magma, approach- 

 ing a diorite in composition. The molecular, syntectic film 

 which is formed by solution along the surfaces of the block 

 must, theoretically, contain equal parts of primary magma and 

 xenolith material. If the former be basalt and the latter a 

 granitoid gneiss, the film must have a dioritic composition. 

 All three kinds of secondary magma — molten globules of 

 gneiss, globule-material dissolved in primary magma as the 

 globule rises, and the material formed in the molecular, syntec- 

 tic film — must be considerably less dense than the primary 

 basalt and rise toward the top of the batholith chamber. A 

 net result of abyssal assimilation is a compound, secondary 

 magma either dioritic or more acid than diorite. 



* Beitrage zur Chemischen Petrographie. II Teil ; Stuttgart, 1905. 

 t The same reasoning applies to xenoliths of normal gneiss immersed in 

 acidified gabbro or diorite magma. 



