220 New York State Museum 



considered as representing the uppermost Cambrian 

 deposits. 



Geology. The Cambrian is the oldest period of the 

 Paleozoic era, generally separated from the older rocks 

 by the most marked unconformity, representing a very 

 long interval of erosion. Except for the few fossils found 

 in the Proterozoic this is the oldest fossiliferous system 

 yet known. At the opening of the Cambrian all known 

 continents were dry land and subject to erosion, and it 

 was during this period that the first great transgression 

 of the seas over the continent occurred. Practically 

 everywhere both in North America and in Europe all 

 Cambrian series of rocks resting upon older rocks begin 

 with a basal sandstone or conglomerate. The deposits of 

 residual sands, the product of long continued erosion, 

 were reworked by the transgressing seas and formed the 

 first deposits, which were generally a pure quartz sand. 

 In cases where the deposits were very thick the seas could 

 only rework the upper portions leaving intact the lower 

 portions with their original structures which may be 

 torrential or eolian. The first sediments of the Cambrian 

 were not always marine, either, for before the transgress- 

 ing sea reached certain areas, as the southern Appa- 

 lachians, great quantities of sands and pebbles were de- 

 posited by the rivers in the form of beach deposits. In 

 their essential features the Appalachian and Cordilleran 

 geosynclines, or sinking areas, were already in existence 

 in the Cambrian, and in general there was an advance of 

 continental seas by shifting of old seas throughout Cam- 

 brian time, though there were also periods of retreat with 

 erosion, followed by advance again in varying directions. 

 The shifting of seas is illustrated by the distribution of 

 the deposits. The Lower Cambrian deposits stretch from 



