REPORT OF THE CHIEF ASTRONOMER 289 



SESSIONAL PAPER No. 25a 



In Table XVII., Col. 2, is entered a typical analysis of the Bitterroot 

 granite, called by Lindgren a quartz monzonite. The chemical resemblance to 

 the British Columbia rock, and a corresponding similarity in macroscopic habit, 

 mineralogical composition (more plagioclase in the Bitterroot granite), struc- 

 ture, and dynamic history suggest the possibility that, in the future, the Kykert 

 granite may be found to be an offshoot of the vast Bitterroot batholith. 



Lindgren states that: — 



' The age cannot be determined with certainty on account of the absence 

 of fossils in the surrounding formations. In the southern part of the batho- 

 lith, near Hailey, on Wood river, it has been shown that the intrusion is 

 certainly post-Carboniferous. As it has been shown that the sedimentary 

 series on the South fork of the Clearwater, near Harpster, is very probably 

 Triassic, a post-Triassic age may, with the same degree of certainty, be 

 attributed to the great granitic batholith.' 



Assuming that the Rykert and Bitterroot granite intrusions were contem- 

 poraneous, we see some reason for referring the Rykert granite to the Jurassic 

 or to a yet later date. However, until further field-work is done in northern 

 Idaho, this correlation must be regarded as quite hypothetical. The Rykert 

 granite may, indeed, be of pre-Cambrian age, though, of course, younger than the 

 Priest River terrane. 



The fact that the Rykert granite is, on the whole, more schistose than the 

 Bitterroot granite is not an argument against their correlation, for it is highly 

 probable that the deformation of the Rykert granite took place when that body 

 was under an exceptionally thick cover. This cover almost surely included the 

 entire thickness of the Summit series, so that this granite lay at a depth of six 

 miles or more before its final shearing, with uplift, began. Under those condi- 

 tions the development of perfect crush-schistosity might be expected even in a 

 Jurassic batholith. 



The thermal or contact metamorphism produced by the Rykert batholith 

 ■was studied only on its west side. There the effects are noticeable for many 

 hundreds of feet from the contact. They consist chiefly in the development of 

 plentiful garnets and of much muscovite and biotite in the schists of the Priest 

 River terrane. The micas form much larger foils than are usual in the schists 

 far from the contact. The metamorphic effects, thus, consist in recrystallization 

 -without the formation of rare minerals. 



Bayonne Batholith and its Satellites. 



North of Summit creek an area of some ten square miles in the ten-mile 

 belt is covered by intrusive basic granodiorite. This mass belongs to the 

 southern extremity of a large batholith which extends northward far down 

 Kootenay lake, and covers a total area of at least 350 square miles. The batho- 

 lith has the form of a rude ellipse about 20 miles long from north to south 

 and 16 miles in greatest width. The Bayonne gold mine is located well within 

 the granitic mass which may, for convenience, be distinguished as the Bayonne 

 batholith. 



25a — vol. ii — 19 



