REPORT OF THE CHIEF ASTRONOMER 761 



SESSIONAL PAPER No. 25a 



magma which enveloped the metallic core of the earth before a true crust was 

 formed. If modern augite andesite is a differentiate from basalt we can similarly 

 regard the possibility that, under certain conditions, bodies of liparitic or 

 granitic magma are the extreme differentiates from the basalt of the substra- 

 tum. The association of andesite with pitchstone and quartz felsite of the com- 

 posite dikes of Arran is one of many occurrences significant in this connection.* 

 The field relations of the average batholith are such, however, as to compel 

 belief in assimilation on a large scale. We seemed forced to believe that the 

 differentiation of syntectics, rather than the differentiation of primary basalt, 

 has produced the greater masses of post-Archean granite. The chemical resem- 

 blance of the average acid pole of this splitting to the primary acid earth-shellf 

 is understood if, in both cases, the anchi-eutectic, granite, separates by liqua- 

 tion and rises. Where sediments only are assimilated, the secondary granite 

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

 sills. 



The longer an abyssally injected and assimilating body holds its fluidity, 

 the more perfect should be the gravitative differentiation. During this active 

 stage lateral fissures or laccolithic spaces may be filled 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 intrusion should be a rock of gab- 

 broid (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 varying in composition — might be repre- 

 sented 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. 



Again, successive batholithic intrusions in the same area should show the 

 same law of increasing acidity with decreasing age. If, for example, a crystal- 

 lized 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 the 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 

 temperature for magmatic splitting— probably an interval of one or two hundred 

 degrees above its melting point:}: — the separation of its acid-alkaline and ferro- 

 magnesian elements by gravity is further perfected. Morozewicz has given a 

 telling experimental demonstration of the process. He melted two pounds of 

 granite and left the superheated melt in a hot part of an active glass-furnace 

 for five days. It was then cooled to a glass. At the end of the time he found 



* J. W. Judd, Quart. Jour. Geol. Soc, Vol. 49, 1893, p. 536. 



t See Cols. 1, 2, and 3, in Table XLIV. 



t F. Loewinson-Lessing, Studien Tiber die Emptivgesteine, p. 380. 



