REPORT OF THE CHIEF ASTRONOMER 543 



SESSIONAL PAPER No. 25a 



The basic and acid dikes were nowhere found in contact. Judging from 

 analogy the diabase dikes would be regarded as younger than the camptonite or 

 than either of the acid kinds of dikes. 



Acid Dikes cutting the Chilliwack Series. 



Apart from the somewhat numerous dikes which are plainly apophysal from 

 the Chilliwack granodiorite (granodiorite porphyry), there are relatively few 

 acid dikes cutting the Paleozoic sediments of the region. The Glacial drift of 

 the valley carries a considerable number of boulders of a porphyritic rock which. 

 judging from the distribution of the erratics, should be in place at several points 

 in the Chilliwack river basin between the lake and Tamihy creek. This rock 

 was actually found in place as a dike or sheet at the 2,400-foot contour on the 

 slope north of the confluence of Middle creek and the river. The exposure is 

 poor and neither the exact relation nor the thickness of the body could be 

 determined. 



This dike-rock is of a light -gray colour, weathering a pale brown, with 

 conspicuous white phenocrysts of oligoclase standing out of the coloured matrix. 

 The phenocrysts measure from 0-5 cm. to 1-5 cm. in length. Much smaller, 

 likewise idiomorphic crystals of quartz and orthoclase can also be seen with 

 the unaided eye. ISTo ferromagnesian mineral could be found either in the 

 hand-specimen or in the thin section. The ground-mass is a finely granular 

 aggregate of quartz, orthoclase, and oligoclase. The rock is a granite porphyry 

 of aplitic composition; it is, however, a rock of very different habit from the 

 aplitic dikes cutting the Chilliwack granodiorite and there is no evidence that 

 the granite porphyry has any direct genetic connection with that or any other 

 of the visible batholiths of the region. 



Basic Dikes and Greenstones in the Chilliwack Series. 



At a few points the great argillite-sandstone series of the Chilliwack river 

 valley and the adjacent region is charged with small bodies of basic and ultra- 

 basic igneous material, all of which is probably intrusive. One of the bodies 

 has the form and relations of a much faulted dike about twenty feet in width; 

 it cuts the sediments close beside the diorite contact on Pierce mountain. The 

 dike has been squeezed and rolled out into a number of more or less perfectly- 

 disconnected lenses. Its compact, dark greenish material proved, on microscopic 

 examination, to be a mass of serpentine, original olivine, and magnetite. The 

 rock was doubtless originally a dunite. At this locality considerable masses 

 of tremolite occur in the sediments and may in part at least have been derived 

 from the serpentine through the metamorphic action of the intrusive Slesse 

 diorite. 



Close beside this dike of altered dunite, the crumpled argillites are inex- 

 tricably mixed with similarly mashed, dike-like bodies of amphibolite, which 

 is transitional in a few places into a fairly coarse-grained gabbro. In the 

 ?abbro the bisilicate has all gone over to an amphibole of actinolitic habit. 



