REPORT OF TEE CHIEF ASTRONOMER 469 



SESSIONAL PAPER No. 25a 



cross-section may have diminished. As with so many other instances, the^ 

 contacts are too meagerly exposed to fix the true alternative. The nephelite 

 syenite was in part injected into the nearly contemporaneous malignite. The 

 common fluidal structure of these rocks also points to a mode of wedge intrusion 

 more like that of dike or laccolith than like that of a stock. The Kruger body 

 may thus represent a composite chonolith, but the problem of its style of intru- 

 sion must remain open. The date of the intrusion was post-Laramie. The 

 alkaline magma may have been squeezed into the schists while mountain build- 

 ing progressed or after it had ceased. The crushing and incipient metamor- 

 phism of this body are on a scale more appropriate to the thrust resulting froms 

 the irruption of the younger Similkameen granite than to the more powerful 

 squeezing effect of the post-Laramie mountain-building. 



True batholithic irruption was resumed in the replacement of schists, nephe- 

 lite rocks, and possibly much of the granodiorite by the Similkameen batholith, 

 This great mass is uncrushed, never shows gneissic structure, and has never 

 been significantly deformed through orogenic movements. 



The composite batholith received its last structural component when the 

 Cathedral granite finally cut its way through Eemmel granodiorite, Similka- 

 meen granite, remnant Paleozoic schists, and possibly through Cretaceous strata, 

 to take its place as one of the most imposing geological units in the Cascade 

 system. The field proofs are very clear that the Similkameen granite was solid 

 and virtually cold before this last granite ate its way through the roots of the 

 mountain range; in the manner shown, for example, in the large intrusive 

 tongues cutting the schist pendant north of Horseshoe mountain (Figure 31)^ 

 The contacts between the two batholiths are of knife-edge sharpness. The 

 younger granite, persisting in all essential characters even to the main contacts, 

 sends powerful apophyses into the older granite, exactly as if the two batholiths 

 were dated several geological periods apart. Both are of Tertiary age and bear 

 witness to the tremendous plutonic energies set free in a late epoch of Cor- 

 dilleran history. Quietly, but with steady, incalculable force, this youngest 

 magma worked its way upward and replaced the invaded rocks. During the 

 same time the satellitic Park granite was irrupted with the stock form and 

 relations. 



Smith and Mendenhall have described a large batholith of granodiorite, 

 intrusive ' into Miocene argillites at Snoqualmie pass in the northern Cascades 

 and 100 miles southwest of Osoyoos lake.* This is one of the youngest batholiths 

 yet described in the world. The more basic phases of the Similkameen batholith 

 present similarities to the rock at Snoqualmie pass. It is thus possible that 

 the Similkameen granite was irrupted in late Miocene, or even in Pliocene,. 

 time. 



The Cathedral granite must be of still later date. In this connection the 

 work of Smith and Calkins is of special interest, for they have found that the 

 Snoqualmie granodiorite is intimately connected with a large body of biotite 



♦Bull. Geol. Soc. Am., Vol. 11, 1900, p. 223; Snoqualmie Folio, U.S. Geol. Survey., 

 1906, p. 9. 



