REPORT OF TEE CHIEF ASTRONOMER 483 



SESSIONAL PAPER No. 25a 



agglomerate and granodiorite is a direct indication that the land surface was 

 flat when the volcanic activity began. 



With the vulcanism or closely following it there was a strong down-warp- 

 ing of the region. Great changes of slope must have occurred, for the agglo- 

 merate is overlain directly by member B, a very thick sandstone essentially 

 made up of the decomposition-products of the granodiorite which were now 

 swept into the down-warp from an uprising area on the east. The resulting 

 accumidation of arkose and feldspathic sands was immense and of long dura- 

 tion. 



Striking characteristics of member B are its massiveness and uniformity of 

 grain and substance. The massiveness is so great that even in large outcrops 

 representing strata fifty feet or more thick, it is often difficult to find the bed- 

 ding-plane at all. In such cases the writer was at first in doubt as to whether 

 the rock were really detrital, so much did it simulate a decolourized granite. 

 Careful search, however, always showed the presence of true bedding which was 

 best displayed in thin partings of dark shale in the sandstone. These shales and 

 sometimes the sandstone itself were found to carry fossil plants; no further 

 question was possible as to the nature of the whole formation. A few ripple- 

 marks were discovered in the upper beds of the member. 



On both sides of the Boundary line the nurnerous readings of strike and 

 dip showed close accordance all across the sandstone through the six miles from 

 the Pasayten river to Chuchuwanten creek. The strike averaged about 

 N. 30° W. ; the dip, about 48° S.W. The apparent thickness of member B is at 

 least 15,000 feet. It is possible, however, that the strata have been in part 

 repeated by a northwest-southeast normal fault running along the valley just 

 east of Monument 81, and it has appeared safer to estimate the thickness from 

 the simple monoclinal element between that valley and the band of agglomerate 

 three miles to the eastward. Even this estimate gives 10,000 feet as the mini- 

 mum. 



At the strong, canyon-like valley of Chuchuwanten creek there is such change 

 of dip (though almost no change in strike) that another normal fault with up- 

 throw on the northeast has been postulated and marked on the map. It 

 is possible that some of the youngest beds of member B are represented only on 

 the southwest side of that fault but they are neglected in estimating the mini- 

 mum thickness of the member as given in the general columnar section. 



The sandstone is normally a light-gray, medium to rather fine-grained rock; 

 seldom showing the bedding-planes in the hand-specimen. It weathers gray to 

 brownish -gray, rarely whitish. Excepting for the rare and thin interbeds of 

 argillite already noted there are almost no variations from the monotonous 

 character of the sediment; no conglomerate was found in this member. The 

 sandstone is well consolidated and is often quite tough before the hammer. A 

 typical specimen was sliced and examined microscopically. As expected from 

 the macroscopic appearance the rock was found to be very rich in feldspar 

 fragments. A rough estimate of the weight percentages credits about 30 per 

 cent to quartz, 30 per cent to orthoclase, 35 per cent to plagioclase (andesine to 

 25a— vol. ii— 314 



