THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XI. — The Mechanics of Igneous Intrusion. (Second 

 Paper) ; bj Reginald A. Daly. 



(By permission of the Director of the Geological Survey of Canada.) 



CONTENTS : 



Introduction _ 107 



Origin and Suspension of Basic Segregations — Bearing on Doctrine of 



Liquidity of Phitonic Magmas 108 



Shattering at Plutonic Contacts 110 



The zone of apophyses 110 



The zone of inclusions ; its origin ._ _ 111 



Shattering by differential thermal expansion _ 112 



Exfoliation at plutonic molar contacts 114 



Illustration from the Madoc-Marmora District of Ontario 117 



Illustration from the Geological Structure near Trail, British Columbia^ 119 



Stability of the Roofs of Magma Chambers 125 



IntroducUon. — In a recent number of tins Journal* the 

 writer has presented some of the facts upon which has been 

 based a hypothesis on the development of the larger magma- 

 chambers now occupied by plutonic rocks. There was entailed 

 in the working out of that hypothesis a brief treatment of a 

 necessary corollary — abyssal assimilation of the formation 

 invaded by stock or batholith. The hypothesis was stated in 

 general terms, Avithout a full discussion of some of the funda- 

 mental postulates and conclusions and without detailed refer- 

 ence to individual areas. In Bulletin 209 of the United States 

 Geological Survey the application of the hypothesis to a rather 

 complex group of stocks occurring in and about Mt. Ascutney, 

 Vermont, has been made, and some degree of assurance 

 attained that this view of intrusion is there of greater value in 

 explanation than are the older theories of intrusion mechanism. 

 * This Journal, vol. xv, p. 269, April, 1903. 



Am. Jour. Sgi.— Fourth Series, Yol. XVI, No. 92.— August, 1903. 



