MODIFIED DRIFT IN PISCATAQUA BASIN. I55 



and can hardly be supposed to be remnants of any formerly level deposit. 

 The beds of clay near Dover point may be of similar origin. 



The clay worked at Plaistow is alluvial, and was deposited in the quiet 

 water, held back by a barrier of kames. The order here is as follows: 

 sand at the top, 5 to lo feet; gray clay, about 10 feet ; gradual transition 

 from gray to blue clay, 3 feet; blue clay, 5 to 10 feet, underlain by sand. 



Beds of gray and blue clay are also found in the high terraces of Mer- 

 rimack river, and in the valley of Hudson river and Lake Champlain (see 

 pp. 94 and 95). In Vermont, where they contain marine shells, they 

 were called by C. H. Hitchcock Champlain clays, from which this name 

 has been applied to the period in which the ice-sheet disappeared. 



The height of these deposits of clay in Hooksett and Pembroke is 

 about 300 feet above the sea ; in Rochester, about 200 ; at Dover Land- 

 ing and Point, 20 to 50; and in East Kingston and Plaistow, about 100. 



The wide distribution and unvarying order of the gray and blue clay 

 suggest that they have some special meaning, which it may be important 

 to understand for the right interpretation of our other records in surface 

 geology. To what conditions and causes do these clays owe their differ- 

 ence in color } * Without being able to explain this, we have presented 

 our observations as fully and plainly as possible. 



Kames and Kame-like Plains about Dover and southward. Near the 

 coast from Dover to Newburyport are frequently found massive kame- 

 like deposits, consisting of high plains or broadly rounded ridges of gravel 

 and sand, which often form water-sheds between wide valleys 100 to 200 

 feet below. The absence in these valleys of the terraces which mark 

 erosion through modified drift, shows that they were never filled with the 

 same materials, and that these remarkable plains and ridges were depos- 

 ited in their present isolated position, with wide areas of lower land at 

 each side. How this took place we can only explain by referring the 



* These clays were the most finely pulverized portion of the abraded material contained in the ice-sheet. Their 

 iron, which is always present in such quantity as to give the burnt bricks a red color, was in the same state of 

 combination as in the rocks before pulverization, principally in ferrous sulphides, silicates, and carbonates. This 

 is its condition in the dark and often blue clayey paste of the lower till. Has the underlying blue clay been washed 

 from the ice-sheet by glacial rivers, transported many miles, and deposited, without the separation and further 

 oxidation of its iron, which would give the clay a gray, brown, or reddish color? and have these changes been 

 partially effected in the overlying gray clay? Or, did these changes take place quite fully before deposition, to 

 be followed by deoxidation through the agency of organic matter, as has been shown by Dawson to be true of the 

 mud now being deposited in the Gulf of St. Lawrence? In either case, the conditions which produced difference 

 in color between the lower and upper portions of the clay remain to be explained. The anomalous section 

 shown in Fig. 38 adds to the difficulty of this question. 



