238 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1915. 



drawal of the oceanic waters from the continental areas during the 

 great period represented by the non-marine deposition of the later 

 Algonkian sediments and the period of erosion preceding the deposi- 

 tion of the superjacent Cambrian sediments, is unknown. It may 

 have been produced by a sinking of the ocean bed that lowered the 

 shore line of all the continents. It was of world-wide extent and of 

 great duration, and it was during this period that the open-sea fauna 

 was presumably first developed in the open ocean, as outlined by 

 Brooks.^ It probably found its way to the littoral zone and de- 

 veloped in the protected waters along the ancient epicontinental 

 shelves. Of this period we have no known record either in marine 

 sedimentation or in life, but I think that the life of the oceans be- 

 came adapted to littoral and shore conditions in Algonkian time 

 during a period when the relation of all the continents to the sea 

 level was essentially the same as at the present time, or possibly the 

 continents may have been still more elevated in relation to the sur- 

 rounding oceans. 



The known fossils contained in the Algonkian sediments of the 

 Cordilleran geosyncline lived in fresh or brackish waters that were 

 rarely in connection with marine waters on the margins of the 

 Algonkian continent of North America. This will explain the 

 abrupt appearance of Beltina, a highly specialized shrimp-like crus- 

 tacean, deep down in the Beltian series. 



When the oceanic waters gained access to the Algonkian conti- 

 nental areas at the beginning of Cambrian time they brought with 

 them the littoral marine fauna which had been developed during 

 the Lipalian sedimentation, and buried its remains in the sands and 

 muds Avhich form the Lower Cambrian deposits. The apparently 

 abrupt appearance of this fauna is to be explained by the absence 

 on our present land areas of the sediments, and hence the faunas 

 of the Lipalian period. This resulted from the continental area 

 being above sea level during the development of the unknown 

 ancestry of the Cambrian fauna. 



I fully realize that the conclusions above outlined are based 

 primarily on the absence of a marine fauna from the Algonkian 

 rocks, but until such is discovered J know of no more probable 

 explanation of the abrupt appearance of the Cambrian fauna. 



ALGONKIAN FORMATIONS. 



The Algonkian rocks are largely formed of mud, sand, gravel, and 

 volcanic rocks that were deposited in lakes, on plains, or in valleys 

 by the action of water, wind, and eruptive agencies. 



1 Brooks, W. K. : The origin of tlie oldest fossils and the discovery of the bottom of the 

 ocean, Jour. Geol., vol. 2, 1894, pp. 455-479. 



