CROSBY, GEOLOGY OF LONG ISLAND 427 



that this far exceeds in extent and continuity the portions of the Tertiary 

 base-level developed by fluvial erosion. In eastern Massachusetts, where, 

 apparently, the exposure to the Atlantic surges was, as now, unrestricted, 

 the Tertiary base-level has a broad and singularly perfect development; 

 but on the coasts of Rhode Island and Connecticut, protected in Tertiary 

 times, as now, by a cordon of islands and reefs, it is rather less distinct and 

 continuous, though by no means wanting. 



The planation of the uplifted and tilted Cretaceous sediments by the 

 Tertiary sea progressed rapidly, developing the well-known unconformity 

 at the base of the Miocene and furnishing, doubtless, the major part of the 

 heavy bed of clay overljdng the Jameco Gravel, which I have elsewhere 

 correlated with the Chesapeake division of the Miocene and which Veatch 

 has correlated with the Sankaty Head deposits of probable early Pleistocene 

 age. This clay is predominantly dark and carbonaceous and abundantly 

 characterized by lignite and segregations of iron sulphide, — characters which 

 seem to forbid its correlation wuth the Pleistocene, and especially with the 

 fossiliferous quartz sands of Sankaty Head. Certainly the fact that it 

 passes downward into gravel containing decomposed granitic pebbles does 

 not demand such correlation. 



When, finally, the Tertiary sea had transgressed over the Cretaceous 

 series and reached the crystalline bed-rock, marine erosion was able, by 

 virtue of the excessively slow subsidence, to accomplish its perfect work, 

 reducing the surface to a plane and the detritus to a residuum of indestruc- 

 tible quartz, which we now know as the "Yellow Gravel" and correlate 

 chiefly, at least, with the Pliocene (Lafayette). The composition of the 

 Yellow Gravel is vastly significant, especially in its genetic relation to the 

 pleneplain; and comparison with the Jameco Gravel should prove fatal to 

 the suggestion of an ultimate glacial origin. Its volume is also impressive 

 and, in view of the limited extent of the Tertiary peneplain, suggests deriva- 

 tion, in part, from the similar gravels of the Cretaceous series. As a result 

 of the progressive subsidence during the deposition of the several Tertiary 

 terranes, we find that in their areal relations the Jameco Gravel is very re- 

 stricted; the Chesapeake Clay is less restricted, and the Yellow Gravel is 

 virtually unrestricted. 



Contrary to the views of several of the later workers in this field, I hold 

 that the Pleistocene glacial history of Long Island is relatively simple. The 

 known facts appear to be satisfactorily accounted for by a single ice invasion; 

 and correlation with the complex Pleistocene stages of the Mississippi 

 Valley is certainly not demanded. 



That the Pleistocene glacial period was, for this region, preceded and 

 ushered in by a long-continued continental uplift is generally conceded, and 



