Vlll 



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 have to consider the 

 still further subsidence of the Champlain 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 

 the barrier of the terminal moraine, where it still remains as the con- 

 necting link between Long Island and Massachusetts. The moraine 

 gave way in places, channels were formed and detached portions re- 

 mained 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 rep- 

 resented by the great submerged ridges which are known as the Nan- 

 tucket and other shoals." 



^"^ 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 thousand to ten thousand years is 

 the latest accepted calculation of the time which probably elapsed 

 since the final recession of the glacier, by one of our most acute and 

 conservative authorities* — a period which is about coincident with 

 the probable time when the area bounded by the twenty-fathom con- 

 tour was above the sea level. It is needless to point out that it also 

 implies no subsequent submergence of the remaining portions of this 

 land since the flora was established. In other words, Long Island, 

 Block Island, Martha's Vineyard, Nantucket, etc., as we now know 

 them, have not been submerged since the final retreat of the glacier 

 and their separation into islands by the submergence of the interven- 

 ing land is a comparatively modern phenomenon, due to the depres- 

 sion and erosion which are actually at work, and which have produced 

 such conspicuous results during the historic period. ' ' 



I have quoted this rather full argument because of its inherent im- 

 portance and because I have felt that I could not better it in any way. 

 We gain from it a twofold conception of the causes which have in 

 part influenced the Long Island Flora and the key to the interpreta- 

 tion of some notable facts, f 



'^Warren Upham, Estimates of Geologic Time, Am. Jottrn Science, 14, 1893, 

 p. 209. 



■j- A. Hollick: Pieliminary Contribution to our Knowledge of the Cretaceous For- 

 mation of Long Island and Eastward. Here is to be found a complete historical 

 discussion of the whole matter. 



