398 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 

 Midway Volcanic Group (in part). 



General Description. — The town of Midway lies well within a large area of 

 basic volcanic rocks which extend along the Forty-ninth Parallel continuously 

 from a point about three miles due east of the town to a point eight miles west 

 of it. On east and west alike the volcanics are bounded by the much older 

 Paleozoic sedimentary complex, so that the lavas may be said to lie in a great 

 syncline-like depression between the Paleozoic rocks of the Phoenix mining dis- 

 trict and the Paleozoios of Anarchist-mountain plateau. Mr. Brock has shown 

 that the Midway volcanics extend for at least fifteen miles to the northward of 

 the Boundary line in the longitude of Midway. It is not known how far they 

 extend to the southward. 



The entire group of volcanics is believed to be of post-Cretaceous age. 

 At several points they rest with apparent conformity on the Oligocene Kettle 

 River sandstones and show no evidence of having undergone the intense crush- 

 ing and profound metamorphism which have affected the Paleozoic lavas in 

 their immediate vicinity. Though of such relatively recent date — late Oligocene 

 or post-Oligocene — the Midway lava formation is considerably faulted and tilted, 

 the older lavas having shared the disturbances that have affected the underly- 

 ing sandstones. 



The upturning has in a measure facilitated the discovery of the stratigraphic 

 relations but it has been found that the exposures within the Boundary belt are 

 too imperfect to declare the whole stratigraphy. Nine different types of lava 

 and several horizons of agglomerate and tuff are represented. These rocks are 

 cut by basic dikes and sheets which have solidified into porphyrites not to be 

 easily distinguished from the compact phases of the extrusives. The structural 

 complexity has been heightened by the injection of many dikes, sheets, and 

 more irregular bodies of syenite porphyry. As a result of these various processes 

 the succession and relative volumes of the different lavas are only partly 

 determined. 



The nine types of lava include olivine basalt, augite andesite, hornblende- 

 augite andesite, biotite-augite andesite, hornblende-augite-biotite andesite, 

 biotite andesite, trachyte, extrusive rhomb-porphyry, and an analcitic lava. The 

 first six species have normal characters and their description need not be 

 detailed ; the last two species named are quite unusual types and will be described 

 at greater length. 



Tuffaceous beds are not rare, though they are subordinate to the massive 

 flows. The pyroclastic phases seem to be most commonly associated with, and 

 composed of, the trachyte. 



Petrography of Suballcaline Lavas. — The olivine basalt occurs on the slopes 

 northwest of Midway and at various other points on the north side of the Kettle 

 river. A notable area of this rock is also found on the rolling plateau between 

 the river and Myer's creek canyon. The basalt has the usual deep gray-green to 

 blackish colour, with phenocrysts of augite, olivine, and, generally, basic labra- 

 dorite. The ground-mass shows the usual variation from the diabasic aggregate 

 of basic plagioclase and augite to the glassy paste cementing microlites of those 



