REPORT OF TEE CHIEF ASTRONOMER 147 



SESSIONAL PAPER No. 25a 



writer would not have the actual, strong belief that the volcanics form a part 

 of one enormous, conformably bedded group upturned in a gigantic monocline. 



Not only the structural relations but, as well, the composition of the breccia 

 illustrates the propriety of regarding both it and the underlying and overlying 

 greenstone as members of this conformable group. The rock is a very massive 

 grouping of angular to subangular, very rarely rounded, fragments of dolomite- 

 marble and of quartzites, embedded in an abundant phyllitic matrix. 



The dolomite is compact and white, weathering the usual buff colour. It 

 is silicious, carrying considerable clastic quartz which is strained and crushed. 

 The specific gravity of a typical fragment is 2-804. Many fragments are 

 highly pisolitic or coarsely oolitic, with grains of excellent concentric structure 

 and of diameters from 1 mm. to 4 mm. The largest dolomite fragment seen 

 was quite angular and measured seven feet by four feet by three feet. 



The greatly sheared matrix is composed essentially of sericite and quartz r 

 the latter often showing typical water-worn outlines. Small rounded grains 

 of dolomite also appear in the thin section. The matrix is a carbonate-bearing 

 phyllite, derived from a clay or mud. No trace of volcanic ash was seen in 

 hand-specimen or in thin section. Notwithstanding the intimate field asso- 

 ciation with true lavas, the whole 200-foot bed must be regarded as a water- 

 laid, though not well sorted, angular conglomerate. Its detrital materials 

 doubtless originated from the Priest River terrane. The specific gravity of 

 a large type specimen of the breccia is 2-824. 



Except for the relatively great abundance of dolomitic material both in 

 the matrix and bouldery fragments of the breccia, the whole rock is extremely 

 similar to coarser phases of the Irene conglomerate. The chief essential 

 difference is that the latter has suffered yet more intense mashing than the 

 200-foot band, which, before the upturning, lay 6,000 feet or more nearer the 

 earth's surface than the basal conglomerate. The amount of shearing and 

 metamorphism in the 200-foot band is intermediate between that shown in the 

 basal conglomerate and that in the similar conglomerate beds of the Monk 

 formation overlying the volcanics. This appears to mean that shearing and 

 recrystallization in similar rocks of the series have, as might be expected, 

 progressed in direct proportion to the depth of their burial. 



Monk Formation. 



The formation immediately overlying the Irene volcanics is, of all the mem- 

 bers of the Summit series, by far the most poorly exposed. Only two complete 

 sections, furnishing even tolerable exposures, appear in the Boundary belt. 

 One of these was crossed on the summits just north of Monk creek but it could 

 not be used as a basis for a description of the typical formation, because most 

 of the beds are there signally metamorphosed by adjacent batholithic granite. 

 The following notes on the formation express the facts which were gathered 

 chiefly on a traverse between Monk creek and the Boundary line along the top 

 of the ridge running east-southeast from Mt. Ripple. Unfortunately, that ridge 

 25a— vol. ii— 10* 



