ECONOMIC GEOLOGY 211 



the last two, the clay is overlaid by the delta deposits of rivers 

 tributary to the Hudson, and the alternation of layers may be due 

 to variations in the flow of the rivers emptying at those points, 

 the sandy layers being deposited during periods of floods. Iso- 

 lated ice-scratched bowlders are not uncommonly found in the 

 clay. 



There is often a sharp line of division between the yellow 

 weathered portion and the blue or unweathered part of the clay. 

 The line of separation between the clay and overlying sand is 

 also quite distinct in most cases. Of the blue and the yellow clay 

 the former is the more plastic, but both effervesce readily with 

 acid,, due to the presence of 3 to 6^ of carbonate of lime, and 

 are, therefore, properly speaking, marly clays. The clay is 

 underlaid by a bed of gravel, sand, hardpan, bowlder, till or 

 bed rock. From Albany to Catskill the underlying material 

 is a dark gray or black sand with pebbles of shale and quartz. 

 The sand grains are chiefly of pulverized shale, the rest being 

 silicious and calcareous with a few grains of feldspar and garnet. 

 This sand can often be used for tempering, but at Catskill con- 

 tains too much lime for this purpose. 



From Catskill northward the clay is in most cases covered by 

 but a foot or two of loam. South of Catskill the character of 

 the overlying material varies. 



THE CLAYS OF THE CHAMPLAIN VALLEY 



The clays of the Champlain valley are estuary formations and 

 of the same age as the Hudson river clays. They underlie ter- 

 races along the lake which have been elevated to a height of 400 

 feet above the lake surface. These terraces may be traced almost 

 continuously from Whitehall, at the head of Lake Champlain, to 

 the northern end of the lake and beyond it, but on account of the 

 extensive erosion which has taken place, they are usually narrow, 

 and it is only at sheltered points, like Port Kent and Beauport, 

 that they are especially prominent. The section involved is yel- 

 lowish brown sand, yellowish brown clay and stiff blue clay, the 

 latter being rather calcareous. The upper clay is somewhat 



