REPORT OF THE CHIEF ASTRONOMER 535 



SESSIONAL PAPER No. 25a 



the Boundary belt west of the Eemmel batholith. The basin of Chilliwack lake 

 has been excavated in this rock, for which the name, Chilliwack granodiorite, has 



been selected. The body has the size and field-relations of a typical batholith. 

 (Plates 44, D, 47, and 62, A>. 



On the Canadian side of the Boundary line the granodiorite underlies at 

 least 100 square miles of mountains. The formation stretches an unknown 

 distance to the northward of the Boundary belt and also continues a few miles 

 on the United States side. 



A couple of miles north of the Boundary line, and a like distance west of 

 the Skagit river, a small granitic stock, of composition probably similar to the 

 more salic phase of the Chilliwack batholith, cuts the Hozomeen series. Owing 

 to bad weather and to other causes, the writer was unable to examine this 

 western slope of the Skagit valley. At his request, Mr. Charles Camsell, of the 

 Dominion Geological Survey, mapped the formations on this slope, and special 

 thanks are due him for this service. He discovered the small stock and has 

 referred its date of intrusion to the Tertiary. As yet the rock has not been 

 studied with the microscope. 



The date of the intrusion of the main batholith can be fixed within certain 

 limits. The granodiorite clearly cuts the greatly deformed sediments on the 

 long ridge northwest of the lake. In that region the strata are unfossiliferous 

 but appear to belong to the same group as the definitely Carboniferous beds west 

 of Middle creek canyon. The granodiorite cuts the Slesse diorite, forming a 

 wide belt of intrusion-breccia with the latter where the main contact crosses 

 Middle creek. The diorite just as clearly cuts fossiliferous Carboniferous slates 

 and limestones. It follows that the granodiorite is of post-Carboniferous date. 

 At no point does it show evidence of crushing or of pronounced straining; as in 

 the case of the older diorite, there can be little doubt as to the relatively late 

 date of the intrusion. In field-habit, as in many essential microscopic details, 

 this granodiorite is like that of the post-Cretaceous Castle Peak stock. There 

 are, thus, some grounds for the belief that the Chilliwack granodiorite was, like 

 the granodiorite at Snoqualmie Pass to the southward,* intruded at a date . as 

 recent as the Miocene. 



In the field the batholith preserves great uniformity of colour, grain, and 

 niassiveness. It was only after microscopic examination that its actual varia- 

 tion in composition became apparent. Three main phases were recognized from 

 the thin sections. 



Petrography. — The most basic phase of the three is a quartz diorite rather 

 than a true granodiorite. It occurs along main contacts and also at points two 

 or more miles from any visible contact ; so that it is apparently not the product 

 of simple contact-basification. A type-specimen was collected at the Boundary 

 line in the lower of the two cirques occurring in the mountains southwest of the 

 upper end of Chilliwack lake. This point is at least two miles from any lateral 

 contact and probably at least a mile from the original roof-contact. The rock 

 is exposed on a great scale on the steep, 4,000-foot slope to the westward of the 



* See page 469. 



