THE AMERICAN MIDLAND NATURALIST. 



to be removed before the island is reduced to sea level is less 

 than that removed in the past. The present form of the 

 island appears to be due to constructive rather than des- 

 tructive agencies. Since the disappearance of the ice sheet 

 there has been some subsidence of the island — about 25 ft. 

 Some think, however, this subsidence may indicate only the 

 washing- through of the lower beaches, or even local sub- 

 mergence. The studies of Johnson, York, and Conard, ('15) 

 and Conard ('24) afford a quantitative basis for the fur- 

 ther study on Long Island of this question from the botani- 

 cal side. 



TOPOGRAPHIC FEATURES RELATED TO STRUCTURE 

 AND GEOLOGICAL HISTORY* 



The. basal deposits of Long Island consist chiefly of Pre- 

 Cambrian gneiss with intrusions of granite, dolomite and 

 gneiss, the latter appearing as ledges on the banks of the 

 East River.** Upon a base-levelled and down warped rock 

 floor, Cretaceous and Tertiary materials were laid down, 

 the line between the deposits of the coastal plain and the 

 metamorphic rocks crossing the extreme west end near Long 

 Island city. The Cretaceous deposits correspond to beds of 

 similar age in New Jersey and southwards. They have not 

 been seen however, in the western half of the island nor 

 have positive equivalents of the Tertiary deposits of the 

 mainland been noted. Only a part of the deposits are true 

 coastal plain deposits, the greater portion of the underlying 

 surface being material of Pleistocene age representing the 

 morainal and outwash accumulations associated with con- 

 tinental glaciers. The Cretaceous is highly developed and 

 possibly comes to the surface at High Hill in Huntington. 



* The account given by Hollick ('93) is followed here. 



** Among the minerals found on the beach of the sandspit at 

 Cold Spring Harbor are red sandstones and shales evidently from the 

 Connecticut Valley lowland, arkose sandstone, quartzite, grey quartzite, 

 quartz schist (slate schist), granite, mica schist, granite gneiss, quartz 

 (rosy, smoky, yellow), conglomerate. Cretaceous clays at Cold Spring 

 Harbor also yield pyrite, limonite and lime concretions. 



For the collection of these minerals I am indebted to Miss Betty 

 Watt, University of Pittsburgh, for their identification to Professor J. 

 W. Goldthwait of Dartmouth College. 



—16— 



