Siliceous lavas are very uncommon in the Tertiary volcanics of the British Columbia interior plateaus, and the present paper records what is believed to be the first such occurrence to be studied in petrographic detail. The rhyodacite here described was found in the unpopulated upper Tranquille plateau area about 25 miles northwest of Kamloops, on the old wagon road from Tranquille to Red Lakes and Copper Creek. The outcrop is an elliptically shaped area 300 feet wide and 500 feet long. Flat-lying basalt flows surrounding it give a steptoe appearance suggesting a volcanic plug.

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