446 A. A. JULIEN — AMPHIBOLE SCHISTS OF MANHATTAN ISLAND 



This general development of epidote in one of the heaviest beds of 

 hornblende schist on the island, with an estimated thickness of at least 

 20 feet, indicates the favorable thermal conditions for generation of the 

 mineral which prevailed during the intense compression and general 

 disintegration of constituents, with partial pegmatization of the rock, 

 which attended the intricate folding and crumpling. 



Another good example of recr3^stallization of new hornblende ma}^ be 

 seen in a short outcrop, about 60 feet long, of epidotic hornblende gneiss, 

 at West One hundred and ninety-eighth street and Fort Washington 

 avenue. This bed, which averages 2 feet in thickness, is bent sharply 

 three or four times in zigzag folds, within a distance of 20 feet on the 

 strike, north 48 degrees east, 85 degrees > southeast. The rock consists, 

 to about 30 per cent, of brilliant plates or blades of jet black hornblende, 

 commonly 3 to 5 millimeters in length, with a predominating ground- 

 mass of gray quartz and white feldspar, with a little browm biotite, minute 

 granules of garnet, greenish white actinolite, and greenish yellow epidote. 

 Much pegmatite and gneiss material have been thrust between the dis- 

 torted laminae of the rock. A large part of the hornblende here appears 

 to be secondary, and the extent of recrystallization suggests a strong 

 tendency toward recovery, during metamorphism, of the granular 

 structure of the original diorite, yet preserved at New Kochelle, Rye 

 and Mam aro neck. 



But at the nearest outcrop of the schist, to the north of this locality, 

 the view that the development of epidote and calcite has not been con- 

 nected with the process of pegmatization finds proof in their entire 

 absence from the extensive and thick bed of dioritic gneiss at the locality 

 on Spuyten Duyvil creek, notwithstanding the abundance of pegmatite 

 throughout the mass in lamination seams and intersecting sheets. Slaty 

 lamination is universal, but few folds occur and no corrugations what- 

 ever, indicating the absence of that intense local compression and per- 

 manent strain which apparently formed the essential condition for epi- 

 dotic alteration. 



With the discrimination of these later and secondary phases of altera- 

 tion we are now prepared to consider the special subject of this paper — 

 the genesis of these crystalline schists, with their burden of hornblende, 

 intercalated as sheets in the Manhattan gneisses, from which that min- 

 eral is otherwise entirely absent. Three hypotheses call for consideration. 



First Hypothesis : Metamorphism of Silicious Sediments 



derivation from aluminous clays or shales 



Of the two classes of metamorphosed seditnents which might possess a 

 basic constitution similar to that revealed by the analysis of the horn- 



