INTRODUCTION 3 



course, any such idea is wholly inaccurate, and the difficulty of 

 determining, in any given instance, which factor has been most 

 potent must be obvious to those who appreciate the complexity 

 of the relationship between these historical and present-day 

 agencies. 



I. Edaphic Factors (Geology of the Range) 

 (a) The Glaciated Region 



5. Perhaps nowhere in eastern North America are there so 

 many features of geological interest as within the area covered 

 by this work. Within forty miles of the city the terminal moraine, 

 the upper edge of the coastal plain, the northern edge of the Cre- 

 taceous deposits all converge. On Long Island is the unique 

 juxtaposition of the coastal plain and the glaciated country. The 

 variety of conditions and immensity of age differences postulated 

 by these facts help to explain the fact that more than 400 species 

 reach their distribution outposts within the area covered by this 

 book. 



6. For the purposes of the phytogeographer the range covered 

 by the work may be divided into glaciated and unglaciated. The 

 extreme southern limit of the several encroachments of the dif- 

 ferent continental ice sheets, known as the terminal moraine, 

 extends, roughly speaking, from Montauk, through Long Island 

 and Staten Island, to upper New Jersey and Pennsylvania. (See 

 map, pi. 2.) Everything north of this line was once covered by 

 ice, varying in thickness from almost nothing near the edge to 

 some thousands of feet in the north towards the centers of glacia- 

 tion. It is obvious that this ice sheet, being approximately the 

 most recent geological phenomenon, nullifies completely what 

 might have been the very considerable effects of the much older 

 geological formations north of the terminal moraine on the vege- 

 tation. Geologically the area north of the moraine is of greater 

 antiquity than anything else in our range; practically, so far 

 as vegetative covering is concerned, it is the most recent, for the 

 recession of the ice is the last major geological phenomenon opera- 

 tive hereabouts. An exception to the statement that the ice- 

 sheet nullifies the older geological formations north of the moraine, 

 are the somewhat extensive limestone areas in the glaciated 



