370 NEW YORK STATE MUSEUM 



of Peekskill creek this affords a striking contrast. It is coarsely 

 crystalline whereas the other is fine; it is very impure with 

 silicates and pegmatites, and has occasional dike intrusions and 

 strong epidotic development whereas the other has none; there 

 is no quartzite on either margin of this Sprout Brook limestone 

 and none beneath it, as may be seen by following up the brook 

 to the point where the limestone disappears, whereas the other 

 has at least 500 feet of quartzite conformably beneath. For 

 more than a mile this crystalline limestone occupies the 

 valley for a width of at least *4 mile. It is well developed 

 for a distance of over 6 miles. It must be several hundred 

 feet thick at the lowest estimate. Farther up the valley, where 

 the limestone disappears, only gneisses of typical Highlands 

 types remain. It is clear that the limestones of these two 

 adjacent valleys can not be the same. The Sprout Brook repre- 

 sentative is much the older. It can not by any interpretation 

 be the equivalent of the Wappinger. But the question still 

 remains as to its relationship to other known limestones. Sev- 

 eral small interbedded limestones occur in the gneisses at points 

 between this locality and the Hudson river at Garrison. Could 

 this Sprout Brook limestone be an unusually large one of them? 

 Its great thickness, the breadth of valley that it fills for several 

 miles, its final disappearance entirely toward the northeast, and 

 its nonappearance anywhere else in the region are considered 

 insuperable objections to that view. On the other hand if this 

 valley be considered a simple syncline pitching gently south- 

 westward then the limestone becomes an overlying formation 

 of the normal Inwood limestone relationships and character. 

 This is the interpretation of the writer. The syncline is too 

 closely folded and too shallow to have preserved any of the 

 overlying Manhattan schist which it is believed belongs with 

 it. If this is the correct identification then the Inwood lime- 

 stone and the Manhattan schist are lower and older than the 

 Poughquag quartzite, and therefore can not be correlated with 

 the Wappinger limestone and Hudson River slates as has pre- 

 viously been done. 



