Nature of batholIthtc intrusions 



369 



sive contact surface through vertical depths of from 300 to 2,200 feet. 

 In every case the contact surface dips away from the granodiorite, plung- 

 ing under sandstone or argillite and truncating the beds. The angle of 

 this dip varies from less than 20 degrees to 80 or 85 degrees (figures 9, 10, 

 11, and 12). On the north side of the granodiorite a section of the 

 domed roof of the magma chamber still remains (figure 12). It is note- 

 worthy that a well developed system of rifts or master joints in the 

 granodiorite seems, with its low dip, to be arranged parallel to the north 

 sloping roof, as if due to the contraction of the igneous rock on losing 

 heat upward by conduction. 



(IB 



7700 



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 Figure 12. — Intrusive Contact between Granodiorite and nearly vertical Cretaceous 



Argillites. 



Sketched in the field, on the north side of the Castle Peak stock, and seen in the wall 

 of a deep canyon near the point "E," figure 7. Granodiorite on right and argillites on 

 left. 



This fact of downward enlargement makes it still more surely impos- 

 sible to conceive that the granodiorite was injected into the sediments by 

 filling a cavity opened by orogenic energy. A visible section even 2,200 

 feet deep does not prove the continuance of downward enlargement with 

 depth ; yet there is no logical reason to doubt that at least the steeper ob- 

 served dips of the igneous contact surface are but samples of its dips for 

 several miles beneath the present land surface. Moreover, if the grano- 

 diorite made its own way through the stratified rocks and was not an in- 

 jected body, passively yielding to ordinary orogenic pressures, there must 

 have been free communication between the now visible upper part of the 



