200 TRANSACTIONS OF THE [APR. 24 



water occupyinfj the trough scooped out by the glacier, wLicLi, 

 in its present depressed and widened condition, we now call 

 Long Island Sound, but which was then a fresh water lake or 

 broad river.* Bearing these conditions in mind we next bave 

 to consider the still further subsidence of the Chaniijlain Period, 

 the re-elevation of the Terrace Period, and the depression which 

 is again going on at the present day. It is evident that at some 

 time during these oscillations of level the sea, having eaten 

 away the coastal plain, finally reached tbe barrier of the termi- 

 nal moraine, where this still remained as the connecting link 

 between Long Island and Massachusetts. The moraine gave 

 way in places, channels were formed and detached portions 

 remained to form the islands which we recognize to-day as 

 Block Island, Martha's Vineyard, Nantucket and the host of 

 other lesser islands which stream out from the end of Long 

 Island towards Cape Cod and the Rhode Island shore, while the 

 eroded portions are represented by the great submerged ridges 

 which are known as the Nantucket and other shoals. 



Whether there were more than one oscillation of level before 

 the final separation was accomplished need not here be discussed, 

 but it is evident that our theory implies the continued existence 

 of land connection, between New Jersey and southeastern New 

 England, by way of Long Island, during a sufficient period of 

 time after the final recession of the glacier, for the pine barren 

 flora to have spread and become established there, and we may 

 even approximate, with some measure of probability, what that 

 period of time may have been. The vast time ratios formerly 

 considered necessary by geologists are gradually but surely 

 giving way to more moderate estimates, and it is of interest to 

 note that from six to ten thousand years is the latest accepted 

 calcidation of the time which probably elapsed since the final 

 recession of the glacier, by one of our most acute and conserva- 

 tive authoritiesf — a period which, as we have seen, is about 

 coincident with the probable time when the area bounded by 

 the twenty fathom contour was above the sea level. It is need- 

 less to point out that it also implies* no subsequent submergence 

 of the remaining i)ortions of this land since the flora was 

 established. In other words, Long Island, Block Island, 



*Dr. Predk. J. H. Merrill concluded, from the distribution of the niorainal 

 material, that the trough of the S( urn d was in existence and lllled with water 

 I)revious to the advent of the glacier [Geolofiry of Long Island, Ann. N. Y. Acad. 

 Sei. iii., :i5'.)J, but I believe that the facts which he nuotes to sul)stantiate this 

 theory are capable of modilicatiou and of beinj? otherwise interpreted. 



t "Estimates of Geologic Time." Warren Upham. (Am. Journ- Sci. xlv., 

 209-220(1893). 



