MINERAL RESOURCES OF THE STATE OF NEW YORK 3OI 



conforms mainly with the dip of a hard quartzite and quartz 

 conglomerate, the Shawangunk grit of Silurian age which disappears 

 in the valley below limestones of the higher Silurian and Devonian 

 formations that likewise are upturned. As Darton states/ " The 

 mountain consists of a widely extended sheet of Shawangunk grit 

 lying on soft Hudson River shales. This sheet lies in a gently west 

 dipping monocline which is corrugated by a series of gentle longi- 

 tudinal folds. To the westward it dips beneath shales and limestones 

 of the succeeding formations in the Rondout valley ; to the eastward 

 it is terminated by long lines of high precipices surmounting steep 

 slopes of Hudson river shales." The monocHnal attitude of the 

 formations, as seen from the northwest edge, represents probably 

 the single wing of a broad anticline, of which the larger part has 

 been eroded off, but of which the southeastern limb is found in the 

 Silurian and Devonian strata that reach in a belt from near Cornwall 

 on the Hudson southwest to Greenwood lake and thence into New 

 Jersey. The interval between the two parallel belts is about 20 miles 



Nature of deposits. The ores occur in fissures, brecciated zones, 

 and along the bedding planes of the grit. Replacement of the 

 latter by the ore-bearing solutions has taken place only to a very 

 limited extent. The main occurrences are within fracture zones, 

 developed no doubt by shearing and slight faulting of the beds 

 during the uplift which came at the close of the Carboniferous period 

 and resulted in the present folded arrangement. The actual fault 

 displacements are slight but are indicated by the grooving and 

 polishing of the walls of the fissures. 



The principal fracturing has been in a plane that crosses the 

 strike of the beds and extends downward at a high or vertical angle. 

 Differential movement, accompanied by fracturing, has taken place 

 also along the bedding planes, and in one instance at least (Summit- 

 ville) has resulted in a brecciated zone of considerable extent. 

 Secondary alteration as an accompaniment of the fracturing is not 

 marked, although there has been an infiltration of white vein quartz 

 and impregnation of the grit by pyrite before the ore deposition 

 took place. 



The ore bodies, as might be expected, are irregular in character, 

 and their size and form are conditioned by the relative degree of 

 fracturing which the wall rock has undergone. In places there has 

 been a minimum of crushing, so that sulphides are restricted to a 

 narrow band or group of individual stringers measured by inches, 



1 N. Y. State Mus. 47tli Ann. Rep't, 1894, p. 541. 



