GEOLOGY AND GEOGRAPHY OF THE CLAY DEPOSITS 499 



Horizontal stratification is usually present, and the layers of 

 clay are separated by extremely thin lamina? of sand. At 

 some localities the layers of the clay are very thin and alternate 

 with equally thin layers of sandy clay. This condition is 

 found at Haverstraw, Croton, Dutchess Junction, Stony Point, 

 Fishkill, Cornwall, New Windsor, Catskill and Port Ewen. At 

 all of the above-mentioned localities except the last two, the clay 

 is overlain by the delta deposits of rivers tributary to the Hud- 

 son, 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 period of floods. Isolated 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 un weathered 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 three to six per cent, of carbonate of 

 lime, and are therefore, properly speaking, marly clays. The 

 clay is underlain 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 



