124 R. A. Daly — Mechanics of Igneous Intrusion. 



of the patent differentiation throws light on the origin of the 

 small plutonic bodies of alkaline granite indicated on the map. 

 The edge of one of these appears in its SW corner. That 

 stock covers an area of about 6 square kilometers north of the 

 international boundary and extends over perhaps as many 

 more south of the line. The mineralogical composition, 

 structure and specific gravity are identical with those of the 

 aplite in veins at the river. The alkaline granite is younger 

 than the tonalite since angular inclusions of the latter are to be 

 found in the more acid rock, which sends strong apophyses 

 into the tonalite-granite at various points. Many wide dikes 

 of the alkaline granite also cut the older basic formations 

 south and west of the batholith tongue. The two granitic 

 types seem to have been differentiated in the deeper levels of 

 a common magma-basin after the upper portion of the magma 

 had consolidated. 



The application of the above mentioned facts to the general 

 problem of intrusion must be made in the light of the deter- 

 minations of specific gravities. These are noted in the follow- 

 ing table, which gives the results acquired from the use of 30 

 specimens taken from the granites and from fair representa- 

 tives of their respective country-rocks. 



Average 

 Rock. Spec. grav. 



Tonalite 2*76 



Hornblende-biotite granite ._ _ S'VG 



Average of batholith ca 2*75 



Lavas of older volcanic series 2*85 



Dioritic rocks associated Avith the last 2 '82 



Banded ash-rocks of older volcanic series .. 2-79 



Peridotite at contact, Columbia River 3-22 



Rock of Basic Stock at Rossland __ 2 '91 



Rossland volcanic series ( aver, of 8 specimens of 



agglomerates, tuffs and flows) 2-85 



Younger, alkaline, granite stocks 261 



It appears from the table that the rock of the main intrusive 

 body, the batholith, is in every case of lower density than any 

 type among the country-rocks exposed in the area. When molten 

 the tonalite-granite would have at ordinary atmospheric pressure 

 a specific gravity of about 2-30*. Under plutonic pressures the 

 same strong contrast of densities between intrusive rock and 

 invaded rocks would persist with but little change. If, then, 

 the magma were thinly molten or even but approximating the 

 condition of perfect fluidity, blocks rifted off from the shat- 

 tered w^all must sink in the magma. The presence of the 

 existing zone of inclusions is only explicable on the supposition 

 * This Jourual, April, 1903, p. 277. 



