Elements of the Paleozoic Platform of JVorth America. 409 



domelike warpings in western Pennsylvania* and southern 

 New York, to which their longitudinal direction clearly corre- 

 sponds. 



The joining of the Appalachian folds that die out in south- 

 ern New York by a new north-south system of folds in eastern 

 New York, brings the folded region close against the Adiron- 

 dack isle and produces another depressed " Vorland," the 

 Champlain basin. The Ottawa-Montreal basin that corre- 

 sponds in its position and also in its form, in surrounding the 

 norih side of the Adirondack isle, to the Lake Superior basin, 

 has also been much encroached upon by the westward pressure 

 of the folded region and no doubt to no little amount by 

 extensive overthrust. 



It will be seen that with the conception here presented of 

 the geologic development of the eastern United States, the 

 Great Lakes fall, by the first impetus to the formation of their 

 basins — omitting the later accessory agencies, as glaciation and 

 pre-glacial drainage-lines — into three groups, viz : 



(a) Lake Superior, oi'iginating from the breaking down of 

 one of the arms of the Canadian shield. f 



(&) Lake Michigan and Lake Huron. Their location and 

 form correspond to the Michigan basin, where they roughly 

 follow the Devonian belts. 



(c) Lake Erie and Lake Ontario, either depressions origi- 

 nating from the action of the Atlantic tangential pressure, or 

 counterparts of later warpings in the upper Ohio basin and 

 western New York. 



We have thus far left out of consideration the Appalachian 

 basin or " geosyncline " which occupies a narrow strip on the 

 west side of Appalachia and is continued northward through 

 New York and Yermont into Canada. It has later become 

 the site of the Appalachian folds. Ulrich and Schuchert;}: 

 have clearly shown that this basin became early subdivided by 

 longitudinal and transverse barriers into a number of smaller 

 basins. In their directions these barriers foreshadow the later, 

 more intensive Appalachian folding, and are early indications 

 of the influence of the pressure acting from the Atlantic basin 

 upon and through Appalachia. It is certain that the Appala- 

 chian basin itself which became the site of the intense folding 

 resulted from the Atlantic pressui^e upon Appalachia, due to 

 suboceanic spread. It is therefore a foreign element, so to say, 

 in the geologic history of the Paleozoic platform, which, how- 

 ever, has strongly obscured the original symmetry of the lat- 

 ter. While all changes here noted on the platform are of 



*See Campbell, M. E., Bull. Geol. Soc. Am., xlv, 277, 1903. 



•f- The Lake Superior basin clearly antedates all the others, at least with its 

 ■western arm, which rests in Algonkian rocks that indicate a very early 

 depression in the Canadian shield. 



t N. Y. State Mus. Bull., lii, 633, 1901. 



Am. Jouk. Sci. — Fourth Skeies, Vol. XXX, No. 180.— December, 1910. 

 27 



