62 Staten Island Association of Arts and Sciences 



hills and underlies the entire region.*' The narrow divide be- 

 tween the lake basin and the lower depression toward the north- 

 east, recently cut through in order to connect the two and thus 

 form a twin reservoir, is a low ridge of the rock, capped with a 

 relatively thin deposit of morainal till. 



The lake, therefore, was a modern topographic feature, geo- 

 logically considered. It had its birth when the continental 

 glacier finally melted and disappeared from this region; and it 

 was subsequently a depository for silt washed in from the sides, 

 for the remains of plants and animals that lived and died in it 

 and around its borders, and for the stray material of all kinds 

 which was thrown into or lost in it since man appeared upon the 

 scene. A vertical section through the lake would, therefore, 

 show soapstone rock at the bottom and extending down to an 

 unknown depth, morainal till varying in thickness from place to 

 place, and silt or peat according to whether the section was 

 toward the middle or near the border. 



Several interesting species of plants have been destroyed by the 

 draining off of the water and the denudation of the adjacent land 

 surface, viz : 



Brasenia purpurea (Michx.) Casp. (water-shield or water- 

 target) has been exterminated on Staten Island by the oblitera- 

 tion of this its only known local station. 



Prunus pennsylvanica L.f. (wild red or pigeon cherry) is also 

 probably eliminated from our local flora, inasmuch as the only 

 known representative of the species on Staten Island was a 

 single small tree on the northwest border, which has been de- 

 stroyed. 



Polypodium vulgare L. (common polypody) was represented 

 by a small patch near Logan's spring — one of the few known 

 localities for this fern on Staten Island. This station is entirely 

 destroyed and will ultimately be covered by the waters of the 

 reservoir. 



Dentaria laciniata Muhl. (cut-leaved toothwort or pepper 



6 See Gratacap, L. P., loc. cit. 8: 3, 4. January 12, 1901. 



