REPORT OF THE CHIEF ASTRONOMER 759 



SESSIONAL PAPER No. 25a 



impossible that these anorthosite bodies are the sites of crnstal foundering which 

 occurred before much assimilation had been accomplished. 



The problem of the anorthosites is clearly as yet one for speculation rather 

 than one capable of final solution. It seems proper to believe, however, that, 

 since all or nearly all of the known anorthosite and gabbroid batholiths are of 

 pre-Cambrian age, they owe their origin to special pre-Cambrian conditions. 

 The stoping hypothesis as a whole expressly relates only to conditions which 

 have characterized orogenic belts in post-Archean time. 



The gabbros of Paleozoic or later age represent bodies either too small or 

 of too low temperature to carry on extensive stoping before their ■■ magmas 

 became rigid. Diorite stocks and batholiths, according to the hypothesis, repre- 

 sent undifferentiated or but partially differentiated syntectic magma — of com- 

 position intermediate between rhyolite or granite and basalt. 



These various considerations incline the writer to the view that the exist- 

 ence 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 LI. 

 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.* 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, approaching a diorite in composition. 

 The molecular, syntectic film which is formed by solution along the surfaces 

 of the block must, theoretically, contain about 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, syntectic 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. 



This reasoning is deductive but it can in some measure be checked by actual 

 observations. Lacroix describes blocks of gneiss up to a cubic metre in size, 

 which have been immersed in molten basalt. By the heat of the lava the blocks 



* The same reasoning applies to xenoliths of normal gneiss immersed in acidified 

 gabbro or diorite magma. 



