GEOLOGY OF THE NORTH CREEK QUADRANGLE 35 



and assimilation hypothesis to the solution of the problem. For 

 full discussions of this hypothesis the reader is referred to Daly's 

 original papers.^ Some of the more fundamental principles are as 

 follows: Batholithic (or stock) magmas have reached their present 

 positions chiefly by the successive engulfment of blocks of country 

 rock (xenoliths) stoped or broken out of the roof and walls of the 

 magma chamber; the xenoliths become immersed and dissolved in 

 the depths of the original magma with the formation of a second- 

 ary magma; when the magma becomes very viscous (due to cool- 

 ing) the xenoliths neither sink nor become dissolved. 



Only a summary of the application of these principles to the 

 North Creek gabbros will here be given, the writer having more 

 fully discussed this matter in a recent paper.^ 



We have showm that the gabbro stocks are of the pluglike or pipe- 

 like form with practically vertical boundaries. Such igneous masses 

 were not intruded by simply displacing or pushing aside the country 

 rock, but rather there was a process of replacement. Thus the 

 mode of occurrence of these stocks furnishes strong evidence in 

 favor of magmatic stoping as an important factor in the intrusion. 



The very presence of the inclusions as xenoliths proves that the 

 process of stoping or rifting off blocks from the chamber vault 

 actually did take place to some extent at least, and this when the 

 magma had cooled to a highly viscous condition and hence had com- 

 paratively little power to stope and too low a temperature to assimi- 

 late the blocks. Thus the occurrence of such xenoliths is quite in 

 harmony with Daly's hypothesis. 



The writer believes that the more acidic patches or masses (al- 

 ready described) within the gabbro stocks are evidence of chemical 

 change within the intrusive magma due to solution or partial solu- 

 tion and diffusion of blocks of country rock. In such cases the 

 magma was just hot enough to melt or partially melt and only par- 

 tially diffuse the blocks of country rock. 



Five or six of the stocks are composed wholly of rocks more acidic 

 than the typical gabbro. In the earlier stage of very active intru- 

 sion the invading magma was more thoroughly molten and as the 

 blocks of country rock were stoped oft* they sank in the magma and 

 became thoroughly dissolved and diffused. Since the country rock 

 was nearly always syenite, granite, or gneiss tlic magma became more 

 and more acidic. 



1 .\mer. Jour. Sci. 1903. I5:26(M)8: Anicr. Jour. .Sci. i(X)3. 16: [07-26; .\iner. 

 Jour. Sci. IQ08, 26:17-50. 



-Jour, of (Icol. 1013, 21:160-80. 

 2 



