REPORT OF TEE CHIEF ASTRONOMER 549 



SESSIONAL PAPER No. 25a 



notably changed by regional metamorphism than are the adjacent Paleozoics. 

 The above-mentioned series, occurring in the Forty-ninth Parallel section and 

 here referred to the Paleozoic, show those degrees of regional metamorphism which 

 characterize the Carboniferous terranes of Alaska, British Columbia, Idaho, and 

 California. At the same time it must not be forgotten that the Forty-ninth 

 Parallel section runs through a part of the Western Geosynclinal Belt of the 

 Cordillera where igneous intrusion has been specially effective. It is, therefore, 

 quite possible that both Triassic and Jurassic beds are included in one or more 

 of the Pend D ; Oreille, Attwood, Anarchist, or Flozomeen series. However, it is 

 probable that, in the areas mapped under the first three names, post-Jurassic 

 sediments do not occur. 



c. With very few exceptions the law holds for the world that batholithic 

 (and stock) intrusion of granites and more mediosilicic plutonic rocks is 

 directly preceded by specially severe orogenic movements. Combining this prin- 

 ciple with that noted under a, it is often possible to obtain valuable indications 

 as to the age of the intrusive bodies so dominant on the Forty-ninth Parallel. 

 Such suggestions can sometimes throw light on the age of associated sediments. 



Similarly with proper caution, one may use the degree of metamorphism 

 of igneous intrusive bodies as pointing with more or less probability to their 

 dates of intrusion. For example, the Castle Peak stock, cutting upturned 

 Upper Cretaceous strata, is certainly post-Cretaceous in age. It is not at all 

 crushed or sheared and seems, therefore, not to have suffered the squeezing 

 which this whole region underwent in the Miocene. (See Smith and Calkins, 

 Snoqualmie folio, U.S. Geological Survey, 1906). The intrusion of this stock is 

 therefore best referred to the later Miocene or Pliocene. As above noted this 

 correlation is confirmed on other grounds. 



6. Lithological resemblances among igneous rocks. — This principle can 

 obviously be used only with great care, but when it is applied along with all the 

 ethers, it can give immediate and valuable results. Many tentative correlations 

 on this basis have been described in the body of the report. 



7. Consanguinity among the igneous rocJcs an aid to correlation. — -To a 

 student of petrology there is little difficulty in believing that probable correla- 

 tions can often be made among adjacent igneous bodies which show, in their 

 composition, that they have been formed by mutual differentiation. They may 

 differ slightly in age but, perhaps with few exceptions, by only a small fraction 

 of a geological period for each group of bodies so compared. At the end of that 

 relatively brief period the igneous complex is frozen, and in most cases the next 

 invasion of magma has different composition and runs through a somewhat 

 different cycle before it too crystallizes. Consanguineous bodies are likely, 

 therefore, to be nearly of the same geological age. Partly on this ground several 

 correlations have already been made ; for example, the Slesse diorite and Chilli- 

 wack granodiorite ; the Castle Peak granodiorite and the adjacent bodies of 

 syenite porphyry; the Similkameen and Cathedral batholiths; the Coryell 

 syenite batholith and the syenite-porphyry chonoliths, dikes, sheets, etc., of the 



25 a— 36 



