E.EPOET ON THK MiNES AND MiNING ExHIBIT. 345 



This condition is found at Haverstraw, Croton, Dutchess Junction, 

 Stony Point, Fishkill, Cornwall, New Windsor, Catskill and Port 

 Ewen. At all the above-mentioned localities, except the last two, the 

 clay is overlain by the delta deposits of streams tributary to the Hud- 

 son, and this alternation of layers may be due to the variation of the 

 flow of rivers emptying at that point, the sandy layers being deposited 

 during layers of flood. The delta of Catskill creek has been found at 

 Leeds, some two miles west of the Hudson river, and the delta of 

 Pondout creek, which flows into the Hudson at Port Ewen, will no 

 doubt be found by following the creek back to the old shore line of the 

 estuary. 



Isolated ice-scratched bowlders are not uncommonly found in the 

 clay. They were probably dropped by icebergs floating down the 

 estuary to the sea. There is sometimes a sharp distinction between 

 the yellow or weathered and the blue or unweathered portion of the 

 clay. 



The line of separation also between the clay and the overlying sand 

 is often very sharp. The blue clay is more plastic than the yellow, 

 but both effervesce readily with acid, owing to the presence of three 

 to six per cent of carbonate of lime, and are, therefore, properly speak- 

 ing, marly clays. 



The clays are underlain by a bed of gravel, till or bedrock. From 

 Catskill northward, the clay is in most instances covered by but a foot 

 or two of soil. South of Catskill the character of the overlying mate- 

 rial varies. 



The thickness of the clays varies as the following figures will show : 



Eddyville Blue clay 10 feet. 



Lefever Falls Blue clay 42 " 



Dutchess Junction Blue clay 13U " 



Eosendale Blue clay 30 " 



Poseton Blue clay 233 " 



Haverstraw Blue clay 150 " 



All the streams tributary to the Hudson have at an early period 

 built extensive deltas, which remain to tlie present day and form ter- 

 races along the sides of the valley, which in many instances, as, for 

 example, at Newburgh and Haverstraw, served as sites for towns and 

 cities. 



To briefly mention the origin of these clay deposits : Subsequent to 

 the retreat of the glacier which covered the continent there was a 

 depression of the land, which amounted to 80 feet at New York 

 city and 240 feet at Albany. During this period a great amount of 

 plastic clay was deposited, produced by the glacial attrition of the 

 shales and limestones to the north. The latter, no doubt, give the clay 

 its marly character. The upper portion of the clay is more siliceous, 

 and overlying it, as already stated, is a bed of sand, indicating a change 

 in the nature of the material waslied into the estuary. During the 

 period of submergence much of the siliceous matter washed into the 

 estuary was deposited at the mouths of the tributary streams, thus f orm- 



44 



