I46 NEW YORK STATE MUSEUM 



lengthened the Eastern Interior basin, and extended it into 

 southeastern New York, or considerably farther north than the 

 opposite East Central basin. Moreover, it may have produced 

 secondary depressions east of the Michigan basin, which have 

 finally found expression in Lakes Erie and Ontario: The prin- 

 cipal facts suggesting the latter view are the general parallel- 

 ism of these lake basins with that portion of the Appalachian 

 folds southeast of them x whence the push came. It should, 

 however, in this connection be taken in account, that it can have 

 been but the last stages of Appalachian folding that produced 

 the gentle down-warping of these basins, since the outwardly 

 convex strike of the earlier Paleozoic formations (best seen at 

 the west end of Lake Ontario) shows that this was an elevated re- 

 gion until at least Devonic time. It is therefore quite possible 

 that these depressions are the counterparts of the late (early 

 Tertiary) domelike warpings in western Pennyslvania 2 and 

 southern New York, to which their longitudinal direction clearly 

 corresponds. 



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 Cham- 

 plain basin. The Ottawa-Montreal basin that corresponds in its 

 position and also in its form, in surrounding the north 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, originating from the breaking down of one of 

 the arms of the Canadian shield. 3 



1 By drawing a straight line connecting the folds from the Tennessee- 

 Virginia line to the Pennsylvania-New Jersey line, one obtains a line 

 that indicates the general direction of this portion of the folds, and that 

 line is parallel to the two lake basins. 



2 See Campbell, M. B. Geol. Soc. Am. Bui. 1903. 14 1277. 



3 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. 



