UNCONFORMITY BETWEEN AVALON TEERANE AND CAMBRIAN 221 



Momable formations, and on Trinit_y bay with the 

 base of the Signal Hill sandstones, the Momable, Tor- 

 bay, and Conception formations. At no point are 

 they shown in contact with the upper beds of the 

 Signal Hill sandstones. The unconformity, like that 

 of the Grand canyon of the Colorado, cuts across 

 the entire Algonkian series, indicating profound oro- 

 graphic disturbance and long continued erosion prior 

 to the deposition of the Cambrian rocks (figure 6). 



Extent of unconformity. — It is impossible, from our 

 present information, to determine the extent of the 

 unconformity between the Cambrian formations of 

 the Avalon terrane, as there is no one section where 

 there is a transition from one to the other. The un- 

 conformity now known proves that a profound oro- 

 graphic movement raised the indurated sediments of 

 the Avalon terrane above sealevel, that the forma- 

 tions were folded and faulted, and that subsequent 

 erosion cut across the entire series from the highest 

 beds of the Signal Hill sandstones to the Archean be- 

 neath. What formations existed above the Signal 

 Hill sandstones and were removed by pre-Cambrian 

 or post-Cambrian erosion is unknown. 



ALGONKIAN ROCKS OF THE LAKE SUPERIOR REGION 



Table of "pre-Camhrian rocks. — The Algon'kian rocks 

 of the Lake Superior region comprise a great series 

 of sediments between the Cambrian and the Archean. 

 The following tabulation (page 222) by Van Hise^ 

 illustrates the order of succession and the unconform- 

 ities between the three upper members of the Algon- 

 kian terrane. 



Keiveenaiv series. — The Keweenawan is now gener- 

 ally recognized as a series many thousands of feet 

 thick, consisting of interbedded lava flows and water- 

 deposited detrital material, derived chiefly from the 

 contemporaneous igneous rocks. The volcanics are 

 predominant in the lower part of the series, the in- 

 terstratifications of the two are most frequent in 



*Bull. U. S. Geol. Survey, no. 86, 1892, p. 195. 



