2 GEORGE V. SESSIONAL PAPER No. 25a A. 1912 



CHAPTER XXVIII. 



GENERAL THEORY OF THE IGNEOUS EOCKS AND ITS 

 APPLICATION. 



Condensed Statement of a General Theory. 



It is convenient to summarize the leading conclusions of the foregoing 

 chapters, as to the origin of the rocks erupted during and since the Keewatin 

 division of pre-Cambrian time. 



Igneous bodies are intruded in two different ways: by displacement of the 

 country-rock; and by its replacement. 



Displacement takes place with two kinds of injection: abyssal and satellitic. 

 Abyssal injection is the prelude of all igneous action of Keewatin and later 

 date. Dikes, sheets, laccoliths, chonoliths, etc., are satellitic injections from 

 abyssally injected bodies. 



Eeplacement takes place in two ways : by marginal assimilation and by stop- 

 ing with abyssal assimilation. In both cases the amount of replacement of 

 country-rock is conditioned by the size of the body and is at a maximum for the 

 greater abyssal injections which preserve direct thermal communication with the 

 basaltic substratum. Marginal assimilation is almost entirely confined to the 

 early, more or less superheated stage of the invading body. Stoping with abyssal 

 assimilation, both in notable degree, must continue much longer, or until the 

 magma at the molar contact is very highly viscous. For this and other reasons it 

 seems that stoping is a much more effective agent in replacement than is marginal 

 assimilation. If the magma be superheated the two kinds of replacement must 

 co-operate. Batholiths and stocks generally represent the upper parts of abyssally 

 injected bodies where their magmas have assimilated the invaded formations. 

 The pre-Devonian (generally pre-Cambrian) anorthosites may possibly represent 

 abyssal injections, which were initially too cool to assimilate any considerable 

 amount of the intruded granites, gneisses, etc. Eeplacement in moderate degree 

 has been carried on by thick sheets of magma, but, in general, bodies satellitic 

 from the abyssal injections are too small to have assimilated large volumes of 

 country-rock. 



Vulcanism is initiated in two ways: by the mechanical opening of fissures, 

 or by gaseous perforations of the roofs of intruded bodies. The largest lava 

 fields have been formed above abyssal injections which reached from the level 

 of the primary substrattim to the earth's surface; in these cases the fissures 

 permitting abyssal injection were continuous with fissures opening at the sur- 

 face, and the lava is generally basaltic. Smaller fissure-eruptions may occur 

 where the roofs of satellitic intrusions are cracked and the lava may be of many 



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