1889.] NATURAL SCIENCES OF PHILADELPHIA. 409 



with perhaps one-tenth of the contour indented to a slope of thirty 

 degrees or more. 



In the precipitous portions the sections were very perfectly shown. 

 There is some clay but the larger portion of material is gravel 

 intermixed with pebbles and boulders of various sizes, the boulders 

 some times rounded and often not. The variety of rock was great. 

 The material evidently came chiefly from the Laurentian. Granite 

 and syenite are probably in the largest quantity and greatest variety. 

 White quartz is abundant ; several varieties of porphyry, porphyritic 

 gneiss, garnetiferous gneiss, mica schist and epidote occur more 

 rarely. 



Careful examination shows that considerable drainage from the 

 island takes place through these bluffs. This was plainly visible in 

 July when the ponds evidently were not at their highest, and he 

 was inclined to attribute much of the erosion to this trivial but 

 constantly acting cause. During times of frost its influence must 

 be considerable. 



In these sections, as a rule, the material is heterogeneous, clay, 

 sand, gravel and boulders being commingled in seeming confusion, but 

 at times there is evidence of a sorting. Thus at Clay Head near the 

 north part of the island, there is a great preponderance of clay, but 

 in the sections may be seen what were evidently the beds of ponds, 

 in which muddy water had, for a time, deposited its clay free from 

 stones, this being afterwards buried in the coarser detritus. He saw 

 nothing whatever organic in any of the exposures. 



These alternations of clay and gravel account for the existence 

 of the numerous ponds which appear to be simply collections of 

 rain water in clayey bottoms, the water rising until a porous 

 stratum is reached through which the water percolates to the ocean. 

 At one place a higher pond had an outlet to a lower and here 

 erosion by running water was apparent, but it was trivial and almost 

 the only one seen, except what might be attributed to excessive rain- 

 fall, such as may be seen along our roadsides after a heavy rain. 

 Some of the grass-covered slopes are very steep and this absence of 

 recent erosion shows the extreme porosity of the general surface. 



What most impressed Mr. Rand was the probable very slight 

 change of surface since the glacial epoch. Contrasting this with 

 the erosion in south-eastern Pennsylvania of rocks so very much 

 harder than the loose sand gravel and clays of Block Island the 

 time within which the erosion of the former must have been pro- 

 gressing would seem almost infinite. 



December 10. 

 The President, Dr. Joseph Leidy, in the chair. 

 Thirty-three persons present. 



