EIvISHA MITCHELL SCIENTIFIC SOCIETY. o/ 



absence may be no evidence of the absence of shore 

 conditions. 



Of the relative position of land and water (and con- 

 sequently of the shore-lines) of pre-Cambrian times we 

 know almost nothing-. Those chang-es, which we call 

 metamorphism, have prog-ressed so far, by virtue of the 

 g'reat ag"e of these sediments and their position at the 

 bottom of the stratified series, that it is extrenieh^ dii£- 

 cult to read in them the conditions under which they 

 were deposited. We are quite sure that there existed, 

 at the beg-inning- of Cambrian times, a land area, made 

 up of pre-Cambrian sediments, lying- somewhere along- 

 the Atlantic coast region of North America. A series 

 of very old, hig'hl}^ altered and disturbed sediments 

 now exist as a land area forming- an almost continuous 

 belt between the Appalachian mountain system and the 

 present Atlantic border from Canada southward to 

 Georgia. This area, which is in the main supposed to 

 be pre-Cambrian in ag-e, is plainly shown b}" its struc- 

 ture to have been once involved in a series of compli- 

 cated mountain building- movements, and was in fact 

 part of a g'reat mountain system. Where this land once 

 rose to mountain heig-hts it is now a low, g-ently sloping- 

 and undulating- surface made up of hills of gently 

 rounded outline, all rising- to about the same height and 

 having in the distance the appearance of a flat country. 

 That is to say it has so long been subjected to the 

 forces of denudation that the mountain ridges which 

 once existed have washed away and finally disappeared, 

 leaving a land surface of low relief and weak topogra- 

 phy. What then became of all the material thus re- 

 moved? It found its v/ay into the borders of the ad- 

 joining ocean and was laid down as sediments, and so 

 to the east, south and west of this pre-Cambrian area 



