GEOLOGY OF THE WESt POINT QUADRANGLE, NEW YORK 63 



folds in Peekskill Hollow. The rock is not so coarsely crystallized 

 as the Inwood and older limestones, but is recrystallized sufficiently 

 to destroy traces of such organic remains as may have been present. 

 As a result, no evidence from that source is available from this 

 material. There are no special peculiarities. 



The rock is quite granular where it is affected by decay or leaching, 

 such, for example, as was encountered in the borings of Peekskill 

 hollow, but on exposed surfaces deformation effects such as crushing 

 and rehealing are brought out prominently by differential weather- 

 ing. The bedding, except in very badly deformed zones, is distinct 

 and easily followed, and as far as observed, there is no contact meta- 

 morphism produced in this type. 



c Hudson River formation. The Hudson River formation is very 

 variable in quality of beds represented. Some of them are very fine 

 grained and were originally muds or silts, which by regional meta- 

 morphism have developed into slates and phyllites. There are no 

 undisturbed beds either of this or any of the other formations and 

 on this account simple shales are not to be found at all. 



Some of the beds were originally lithic sandstones which have 

 become quartzites and graywackes and shaly sandstones. The pet- 

 rographic variety is as great as the whole range of such mixtures 

 could be. The principal types are represented by the following: 



(i) A graywacke. This is a greenish rock of granular habit 

 having all the general appearance of the so-called bluestones of the 

 Catskills. This is not the formation, however, to which the true 

 bluestones belong, although it has furnished similar structural 

 material for local use. 



The chief interest attaches to petrographic habit and composition 

 which show that the rock is made up largely of fragments of rock 

 instead of mineral fragments. Many varieties of rock are represented 

 in these constituent grains, the commonest being fragments of rock 

 of types not very unlike some of the beds of the Hudson River 

 formation itself ; that is, slaty and gritty and granular rocks of 

 moderately metamorphosed condition. Fragments of dolomite are 

 sometimes seen also and of course simple grains as well. 



The fact that the formation is made up largely of matters that 

 represent older formations of very much the same type, formations 

 wliich must have been metamorphosed at the time that the Hudson 

 River formation was being accumulated, is very significant. It is 

 in this respect similar to the true bluestones of the Catskills 

 which are made up chiefl}- of lithic grains believed to represent in 

 that case derivation from the Hudson River formation and its asso- 



