220 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



pillow-structure observed in the Lewis range, and the extraordinary size of the 

 feldspar phenocrysts in the porphyritie phases. The individual flows were 

 generally of great thickness, reaching as much as 400 feet. 



From the close association with basaltic dikes and sills cutting the Siyeh 

 (Kitchener) formation, it is believed that the feeders of the fissure eruptions 

 can be actually seen. The eruptions began at the close of Siyeh time but were 

 intermittently continued through the Sheppard and early Kintla times. Fol- 

 lowing the correlation of the preceding chapter, all of this vulcanism falls 

 within the Middle Cambrian period; the Purcell Lava proper, underlying the 

 equivalent of the Flathead sandstone, seems to belong to a period of c-rustal 

 Assuring which accompanied the widespread Middle Cambrian subsidence. 



