Merrill Metamorphic Strata qf S. K N&w York. 889 



of quartz and biotite or hornblende. Occasionally, layers of 

 pure biotite schist, an inch or two in thickness, are intercalated 

 with white, coarsely granular quartzite. This rock forms the 

 summit and eastern Blope of the ridge which separates the 

 Sawmill River valley from that of the Qudson and also occurs 

 in that on the oast side of the Hudson between Yonkers and 

 Spuyten Duvvil. it forms, as well, the anticlinal ridge of 

 Fordham Eeights which borders the east shore of the Harlem 

 River and of which the southern extremity forms the long 

 narrow hill by the northern end of Seventh avenue and which 

 separates the latter from the Boulevard. 



In general terms this quartzite gneiss is the gray rock used 

 for a building stone in southern Westchester County. The 

 Yonkers gneiss is also used for building, hut not so extensively 

 and is characterized by its reddish color. No section has vet 

 been found which shows accurately the thickness of this gray 

 gneiss. It is at least two hundred feet thick and, in many ex- 

 posures, has an aggregate thickness of half a mile, but in these 

 it is apparently repeated many times by folding. As in most 

 cases the folds are isoclinal and their arches have been removed 

 by erosion, there is little stratigraphical evidence of the thick- 

 : the stratum thus folded. 



Since this rock is well exposed and its stratigraphical rela- 

 tions are well shown in the Fordham Heights it is proposed to 

 call it the Fordham am ws. 



The Fordham gneiss varies widely from the normal type, in 

 places, through the presence of hornblende and garnet and an 

 increase in the amount of feldspar and mica, but the localities 

 of variation are comparatively few. 



Between the Fordham gneiss and the crystalline limestone 

 of the Hastings Quarry and in Yonkers at the south end of 

 the railroad trestle near the Lowerre Race Track, a stratum of 

 thinly bedded quartzite from five to ten feet thick is found. 

 It seems probable that this is of wide distribution but on ac- 

 count of its slight thickness it is very rarely preserved when 

 the limestone which rested upon it has been removed by 

 -ion. 



At Tuckahoe this quartzose stratum next beneath the lime- 



ue contains numerous flakes of biotite. 



The position and stratigraphy of the limestone areas of 

 Westchester County have been carefully studied by Professor 

 Dana. Biy only contribution to the geology of these beds has 

 been to determine the relations and character of the associated 

 rock-, to note a few unrecorded outcrops and in a few cases to 

 extend the limits of those previously known. 



Professor Dana has estimated the thickness of the bed oc- 

 curring in Tremont and the Harlem Riven- valley at from six 



