Wright.] 



58 [December 20, 



series of ridges we have described, and their passage in many in- 

 stances over large "lenticular " moraine hills, would forbid the appli- 

 cation of such a theory here. Professor Agassiz thought those in 

 Maine were formed in some way under the glacial sheet by the irreg- 

 ular pressure of the icy mass. But the absence of scratched stones 

 is conclusive against that theory. 



The large " lenticular" (lens-shaped) hills, of which Prospect Hill 

 in Andover, Brown's Folly in Danvers, Pingree's Hill in Topsfield, 

 Bald-pate Hill in Georgetown, Golden Hill in Haverhill, and the 

 islands in Boston harbor, are typical, abound in Essex County, Mass., 

 and in Rockingham County, N. H., and to a less degree farther 

 inland. These are marked wherever they are known to occur in the 

 space included in Plate III. They are, as will be perceived, conspic- 

 uous for their absence in various places. Without doubt, these 

 remarkable hills, though ranging from 200 to 300 feet in height, be- 

 long to the "till," or ground moraine; for the pebbles and boulders 

 in them are scratched, and are mingled with clay which is densely 

 packed, and without stratification. These peculiarities were well 

 seen in the construction of the Lawrence reservoir, which is situated 

 near the top of such a hill, upon which the Andover series of ridges 

 rested as a superficial deposit, and at a height of about 160 feet 

 above the intervale on the Merrimack River, at that point. A fact 

 which I had observed, but was unable to account for, is thus ex- 

 plained to me by Mr. Upham. He however gives credit to Professor 

 Torell 1 for the idea that the dark (usually blue) color of the lower 

 members of the " till," was due to seclusion from the air. The 

 interior portion of these " lenticular " hills is of a dark blue or gray 

 color; while with tolerable uniformity, for a depth of fifteen or 

 twenty feet, so Prof. Hitchcock and Mr. Upham inform me, the 

 material is of a reddish hue, and to this depth the stones are usually 

 more angular and less worn than below. Why these hills should 

 assume the shape that characterizes them, is more than I can sur- 

 mise. But on the supposition that they were formed under the gla- 

 cier, the present covering of less rounded and more oxidized mate- 

 rial, is easily accounted for. This material was precipitated upon 



1 See abstract of paper read at the Buffalo Meeting of the American Association, 

 1S76, on " Glacial Phenomena in North America," by Otto Torell, published in 

 New York Times, and in Am. Jour. Science and Arts for January, 1877. 



