636 



NEW YORK STATE MUSEUM 



St Croix inva- 

 sion and birth 

 of Missis- 

 sippian sea 



St Lawrence 

 channel 



ing Lower Cambric time extended from Alabama northeast 

 to Labrador. It was evidently a synclinal fold — perhaps it 

 would be better to call it a synclinorium — within the south- 

 eastern border of a very large Algonkian continent. After the 

 cloise of the Lower Cambric, this old continent seems to have 

 been subjected to a slight elevation, whose effect, however, made 

 itself manifest principally in the eastern part. Here the Appala- 

 chian trough was almost drained of its sea, which appears 

 further to have been confined to the western half of the original 

 trough by the emergence of a fold. 



However, long before the close of the Middle Cambric, as at 

 present defined, a second period of subsidence set in, submerg- 

 ence beginning along the east side of the Rocky mountain pro- 

 taxis, and spreading northeastward. This submergence, which 

 might be appropriately called the St Croix invasion, marks an 

 important event in the development of the present continent, this 

 being nothing less than the birth of the great interior continen- 

 tal sea, to which Walcott 1 has applied the term Mississippian. 

 This sea, sometimes almost oceanic in extent, continued, with 

 some interruptions and more frequent modifications of its out- 

 line, through all Paleozoic time. In Mesozoic time it was greatly 

 reduced and restricted practically to the Great Plains region. 



With the beginning of the Upper Cambric, which, how- 

 ever, had been preceded by a period of partial reemergence 

 and at least local erosion of the old Lower Cambric land, 

 specially in the southwest, the new sea had transgressed 

 beyond the Adirondacks and soon thereafter probably effected 

 communication with the Atlantic by way of the northern end of 

 the restricted Appalachian trough, or, as this portion might be 

 more appropriately called, the St Latvrence channel. 2 This 

 communication with the Mississippian sea either continued 

 through the LTpper Cambric and the following Beekmantown 

 age, or it was interrupted and revived again in the latter time. 

 Only once thereafter, i. e. in the Utica age, did it serve the same 



1 Am. ass'n adv. sci. Proc. June 1894. 42:129-69. 



2 Mr Matthew finds this Dicellocephalus fauna common to America and 

 Europe. See Trans. Royal Soc. Canada, 1893, v. 10, § 4, p. 11. 



