250 



SCIENCE. 



[N. S. Vol. VIII. No. 191. 



cott that a great orographic movement, fol- 

 lowed by long-continued erosion, took place 

 between the Archean and Algonkian ages. 

 Taking as an example of the nature of 

 the Algonkian changes one region alone, 

 the Lake Superior region, where the strati- 

 graphical record is more complete, we have: 

 1, the Lower Huronian schists, limestone, 

 quartzites, conglomerates, etc., with their 

 eruptives, closely folded and attaining a 

 maximum thickness of probably over 5,000 

 feet. 



2. The Upper Huronian, unconformable 

 to the Lower, a series of more gently folded 

 schists, slates, quartzites, conglomerates, 

 interbedded and cut by trap, with a maxi- 

 mum thickness of 12,000 feet. In the 

 Animikie quartzites of this age have, 

 according to Selwyn, been detected a track 

 of organic origin, in the Minnesota quartz- 

 ites Lingula-like forms, as well as obscure 

 " trilobitic-looking impressions ; while car- 

 bonaceous shales are abundant." 



3. Between these Huronian rocks and 

 the true Cambrian series are interpolated 

 the Keweenawan clastic rocks, with a maxi- 

 mum thickness of 50,000 feet. Though 

 these beds are by some high authorities re- 

 ferred to the Cambrian, the fact remains 

 that this series, whether Cambrian or 

 Algonkian, is unconformable to the Huron- 

 ian, and composed of fragmental rocks, the 

 upper division being 15,000 feet thick, and 

 consisting wholly of detrital material largely 

 derived from the volcanoes of the same 

 series. Between each series is an uncon- 

 formity representing an interval of time 

 long enough for the land to have been raised 

 above the seas, for the rocks to be folded, 

 to have lost by erosion thousands of feet, 

 and for the land to sink below the surface 

 of the ocean. 



Again, between the Precambrian and 

 Cambrian there was, according to "VValcott, 

 a great uplift and folding of rock, succeeded 

 by long-sustained erosion, over all the con- 



tinental area. It was not, however, he 

 states, ' as profound as the one preceding 

 Algonkian time, as is proved by the more 

 highly contorted and disturbed Archean 

 rocks beneath the relatively less distui-bed 

 Algonkian series.' * 



The evidence of the existence of life- 

 forms in the Huronian and Keweenawan 

 times is indicated by the presence of thick 

 beds of graphitic limestone, beds of iron 

 carbonates, and by a great thickness of car- 

 bonaceous shales, which are represented by 

 graphitic schists in the more altered strata. 

 In the Animikie rocks on the northern 

 shores of Lake Superior, Ingalls iinds abun- 

 dant carbon, and it is said that in certain 

 mines and openings rock-gas forms to a 

 considerable extent. Also small quantities 

 of rock may even be obtained which will 

 burn. " These substances must result from 

 the ordinary processes which produce 

 rock-gas and coal in the rocks of far later 

 age. The hydrocarbons which occur so 

 abundantly in the slightly metamorphosed 

 shales of the Huronian about Lake Superior 

 must be of organic origin," and, if so, the 

 graphitic schists of the same system " are 

 in all probability only those hydrocarbona- 

 ceous shales in a more altered condition." 



As to the fossils actuallj^ detected in what 

 are by some geologists regarded as Algon- 

 kian strata, Winchell has detected a Lingu- 

 la-like shell in the pipestones of Minnesota. 

 Selwyn has described traces of animals 

 in the Upper Huronian of Lake Superior. 

 Murray, Howley and Walcott have discov- 

 ered several low types in the Huronian of 

 ^Newfoundland ; i. e. , a mollusc (Aspidella 

 terranoviea)\ and traces of a worm {Aren- 



* The North American Continent during Cambrian 

 Time. Twelfth Ann. Eep. XJ. S. Geological Survey, 

 p. 544. 



fDr. G. F. Matthew writes me as follows regard 

 ing this supposed fossil: "I have seen Aspidella 

 terranovica in the museum at Ottawa and doubt its 

 organic origin. It seems to me a slickensided mud- 



