REPORT OF THE CHIEF ASTRONOMER 491 



SESSIONAL PAPER No. 25a 



uncrushed Castle Peak granodiorite. These facts suggested in the field that 

 the diorite intrusions are of older date than the Castle Peak stock. Micro- 

 scopic study has, however, shown much similarity between the diorite and the 

 basified contact shell of the larger stock, so that a nearly contemporaneous 

 origin of all three bodies seems possible. On this second hypothesis the orogenic 

 stress responsible for the shearing of the diorite might be regarded as having 

 been local and connected with the profound Lightning creek fault or with the 

 intrusion of the slightly younger Castle Peak granite; or the stress might be 

 considered as having been more wide-ranging and strong enough to shear the 

 smaller bodies but too feeble visibly to affect the much larger stock. No facts 

 were observed which would enforce a decision between these two hypotheses 

 though the former seems the more probable. It is known that all three bodies 

 are of post-Pasayten (post-Laramie) age and almost certainly of pre-Pliocene 

 age. If, as seems probable, the Castle Peak stock dates from the mid-Miocene, 

 the diorite is perhaps also best referred to the Miocene. 



Petrographically the two diorite bodies are alike. Each shows a conspicu- 

 ous, basified contact-shell. Each shell is rich in femic constituents and averages, 

 for each body, about 100 feet in thickness. The great bulk of each body is a 

 hornblende diorite bearing accessory orthoclase. The rock is of a light gray 

 colour and of medium grain. The essential minerals are: zoned plagioclase, 

 averaging acid labradorite near Ab 4 An 3 , and a highly idiomorphic, green horn- 

 blende. The accessories include a little apatite with titaniferous magnetite, 

 abundant titanite, and some interstitial quartz- and orthoclase. Pyrite is 

 present but may be secondary, like the abundant calcite, chlorite, and kaolin. 

 Biotite seems to be entirely wanting in this principal phase. 



The structure is the normal eugranitic; under the microscope the minerals 

 are seen to be strained but, in all the thin sections examined, are surprisingly 

 free from granulation. The specific gravity of a type-specimen of this phase is 

 2-763. 



The basic contact-shell is composed of a darker gray diorite with much 

 more hornblende than in the principal phase and with a moderate amount of brown 

 biotite among the essentials. The feldspar averages labradorite, Ab t An t ; 

 orthoclase seems to be entirely absent. The accessory minerals are the same as 

 in the principal phase but are somewhat less abundant. Epidote and zoisite are 

 here added to the list of secondary materials. The structure is again the 

 eugranitic. The specific gravity of a type-specimen is 2-832. This phase is a 

 basic hornblende-biotite diorite. 



Other Basic Intrusions Cutting the Pasayten Formation. 



At the Boundary slash near the divide, the younger Pasayten sandstones are 

 cut, at one horizon, by a prophyritic mass which follows the strike of the 

 beds and measures thirty-five feet or more in width. It may be a sill or a dike, 

 the outcrops not sufficing to fix the alternative. The rock is dark greenish-gray 

 and highly porphyritic, with abundant large, white phenocrysts of plagioclase 



