Io6 NEW YORK STATE MUSEUM 



cordance between the present topography and relations and those of 

 Precambric and early Paleozoic time. 



The Precambric gneissic floor appears to have behaved in a 

 measure as though it had no load. It was twisted and broken into 

 blocks like a piece of glass and thrust up into the overlying forma- 

 tions, the force of the shove diminishing to the northwest. The 

 plateau type of the Highlands is primarily the result of upward 

 thrust as a mass and secondarily the effect of the resistant quality 

 of the Precambric rocks when subjected to erosion. The present 

 topographic level of the Fishkill limestone Avould appear to indicate 

 a normal position for the limestone now. Primarily, however, it is 

 a faulted up thrust block ; erosion has exposed the older stratigraphic 

 series which were thrust up into the overlying slates. 



The northern valleys of the Highlands represent down-faulted 

 masses of the younger rocks which later erosion cycles discovered 

 and removed. 



As superstructures on these basal features are forms connected 

 with folding and breaks along the strike and dip. 



Influence of the petrographic character of the rock. The low 

 average level of the Hudson valley is attributable to the ease with 

 which the slates are broken up and removed. The relatively low 

 topographic level of the Fishkill limestone, corresponding with its 

 lower stratigraphic position, is deceptive. In this case, the removal 

 of the slates and the erosion of the limestone obscures the structural 

 position brought about by faulting. 



The present altitude of the high ridge forming " Illinois moun- 

 tain " is due in part to the resistant character of its grits. 



The resistant quality of the metamorphosed rocks in the eastern 

 part of the quadrangle has been a factor in producing their present 

 relief. 



ECONOMIC GEOLOGY 



The agricultural industry. The agricultural interests are 

 chiefly those of fruit growing and dairy farming. The former is 

 conducted on an intensive plan on the hilly land west of the Hudson 

 where well-drained hills of tilted slates, covered with a veneer of 

 till and' coarse gravel, afford highly suitable soil conditions for 

 growing fruit of excellent quality. Large consignments of peaches, 

 apples, pears and small fruit are sent to New York city and New 

 England markets and some growers find a highly profitable business 

 for fancy fruit in the markets of England. Grapes are also a suc- 

 cessful and important crop. 



