REPORT OF THE CHIEF ASTRONOMER 

 SESSIONAL PAPER No. 25a 



499 



The Castle Peak plutonic body thus appears to be a typical stock, an 

 intrusive mass (a) without a true floor, (b) downwardly broadening in cross- 

 section, and (c) intruded in the form of fluid magma, actively, though gradually, 

 replacing the sedimentary rocks with its own substance. It is the most ideally 

 exposed stock of which the writer has any record. 



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Figure 40. — Intrusive contact between granodiorite and nearly vertical 

 Pasayten argillite. Sketched in the field, on the north side of the Castle Peak 

 stock, near point marked "E ' : in Figure 35. Granodiorite on the right. Figures 

 show elevations in feet and dips of contact surface. 



Intrusion op Syenite Porphyry. 



At Monument 80 the Boundary slash crosses a small mass of hornblende- 

 biotite syenite porphyry. It is intrusive into the Pasayten sandstones. The 

 area of the body as exposed on the present erosion-surface is about one-half of 

 a square mile. The contacts are very poorly displayed and it was found 

 impracticable to determine the structural relation of the porphyry. In ground- 

 plan the body is elliptical, with its longer axis directed N.W.-S.E. This ele- 

 ment of form suggests that the underground relations are those of a true stock, 

 but the steady persistence of a strongly porphyritic structure at all points in 

 the body tends to show that it is an injected, rather than a subjacent mass. If 

 this second view is correct the body must be classed among the chonoliths 

 rather than among the laccoliths, for it cuts across the edges of the sandstone 

 beds along the whole length of the intrusive body. 



25a— vol. ii— 32J 



