362 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



Apophyses. — Finally, a more acid phase of the batholith is represented in 

 the numerous apophyses cutting the Rossland volcanics all about the batholith. 

 These offshoots have the composition of an alkaline syenite porphyry, which is 

 very poor in the femic (phenocrystic) minerals, hornblende and biotite. The 

 plagioclase (basic oligoclase, near Ab 2 An 3 ) is here quite subordinate to the 

 alkaline feldspars. The accessories are those of the monzonite but are in very 

 small amounts. The specific gravity of a type specimen is only 2\-601. 



The apophyses are clearly not direct injections of the basified magma now 

 found in crystallized form just inside the main contact-line. They are differen- 

 tiates of the main mass of magma and are analogous to the familiar aplites given 

 off by granitic intrusive bodies. The differentiation was doubtless aided by the 

 special abundance of magmatic fluids, which lowered the viscosity, so that these 

 feldspathic dikes have run out many thousands of feet from the main contact. 

 The strong mineralization often observed in the traps cut by these apophyses 

 may be connected with the act of expelling the fluids during the crystalliza- 

 tion of the porphyry. 



Contact Metamorphism. — The batholith has exerted notable mechanical and 

 thermal effects on its country-rocks. The traps of the Rossland volcanic group 

 have been converted into hornblende-quartz schist and hornblende-biotite-epidote 

 schist. These rocks, with their schistosity-planes directed peripherally about 

 the batholith, can be traced through a distinct exomorphic zone from three to 

 six hundred yards or more in width. In the old sediments of the Sutherland 

 complex, andalusite has been liberally developed, but it is not easy to say how 

 far the metamorphism of those rocks was due to regional processes, and is thus 

 of older date than the batholithic intrusion. 



Syenite and Granite Porphyries Satellitic to the Coryell Batholith. 



Along the southern border of the Coryell batholith a long area some two 

 square miles in extent has been mapped as underlain by syenite porphyry. 

 This body has every appearance of being simply a late differentiate of the 

 Coryell syenite, a mass which has been injected along the contact of the main 

 batholith with the volcanic rocks. The porphyry is younger than the coarse 

 syenite, since it encloses blocks of the latter and sends tongues into it. Yet 

 the mineral components of the two bodies are similar. The porphyry has 

 phenocrysts of augite, sometimes of brown hornblende, as well as of biotite, 

 alkaline feldspar, and a few soda-lime feldspars. The ground-mass is essentially 

 like that in the chonolithic rock about to be described; in fact, these two rocks 

 are so alike in mineralogical and chemical constitution that the account of the 

 chonolithic rock, which has been chemically analyzed, will serve for both. The 

 principal difference between them consists in the fact that the larger body has 

 a coarser grain. 



South and southeast of the batholith there are very numerous dikes and 

 one irregular intrusion (chonolithic in its relations) which respectively cut 



