GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 8l 



others west. The strike remains practically unchanged for some 

 distance in many instances for beds with the same general dip. The 

 limestone differs from the slate at the north with its isoclinal 

 arrangement over wide intervals. 



The tendency to overthrust, shown in the section along the rail- 

 road, probably prevails over the entire area. Strike faults are most 

 apparent. The regularity of the strike for long distances seems to 

 indicate that horizontal offsetting has not been important. Two 

 large breaks along the strike are shown on the map. They are 

 shown in the field by long stretches of swampy lowland that may 

 be followed for several miles across country. They seem to be the 

 northward projection of faults in the Highlands. The presence of 

 these large breaks and minor ones complicates the question of the 

 age of the limestone. The displacement must have been a large 

 one at places as, for example, along the fault line that bounds 

 Shenandoah mountain on the east. Possibly beds of very different 

 age lie in close proximity. 



Metamorphism and alteration. Were it possible to trace con- 

 tinuously from west to east the beds now known to be present along 

 the western margin of this limestone, more could be definitely 

 determined about gradation in metamorphism to the eastward. 

 Examination of the belt has shown that the rock usually displays 

 greater crystallinity as one goes eastward. Banded limestones not 

 very different from some seen in the Dover valley were noted near 

 Gregory's mill. There is much evidence of crushing. Bunches 

 and veinlets of calcite and quartz nests and stringers are abundant. 

 These indicate hydrothermal activities. Organic remains have 

 doubtless been obliterated by these as well as by crushng, shear- 

 ing and pressure. 



Summary. The Fishkill limestone, in its relations to the 

 slates, stands essentially as a huge faulted block. Though less 

 plainly shown, the same is true of its relations to the Highlands 

 mass. This arrangement has produced a northwestward gradation 

 by faulted blocks from older to young masses. These considera- 

 tions afford further reason for believing that the Highlands owe 

 their present elevation to the mountain-making processes that gave 

 birth to the Green mountain system and that the younger rocks 

 once had an extension much to the south of their present southern 

 limit, thus giving an altogether different notion of the early rela- 

 tion of the Paleozoic sea to the Precambric land from that which 

 the present topography might be assumed to show. 



