216 ANNALS NEW YORK ACADEMY OF SCIENCES 



The thin seams of cyanite schist described as occasionally occurring in 

 the mica schist on Manhattan Island are sufficiently different from the 

 ordinary mica schist to deserve further mention. Eeferring back to 

 analysis 5, p. 212, which is of such a schist, it will be seen that it is made 

 up almost entirely of silica and alumina. If this schist had been derived 

 from an original sediment, it would mean that there must have been a 

 very thin layer of practically pure kaolinite and quartz where the thin 

 seams of cyanite schist now occur. As already mentioned, it hardly ever 

 occurs in seams over one or two inches wide. It is not very probable that 

 such a remarkable concentration of these two constituents should occur in 

 such narrow seams when the surrounding sediments were of such entirely 

 different composition. What seems more probable is that some of the 

 more soluble original constituents have been leached out by percolating 

 waters along these seams, leaving behind the less soluble alumina prob- 

 ably present in the form of kaolinite. The stringers of introduced quartz 

 associated with these seams would seem to bear out this theory. They at 

 least indicate that such circulation has taken place. This circulation of 

 water along these seams and the introduction of quartz was probably 

 closely related to the pegmatitic intrusions occurring in the schist which 

 are discussed later. 



Associated Igneous Rocks 



' • foliated basic intrusions 



Hornblende Schist 



Intercalated with the mica schist of the Manhattan formation are often 

 layers of hornblende schist which vary in width from less than a foot up 

 to two hundred feet or more. These layers may often be followed along 

 the strike for several thousand feet. Sometimes several of them will 

 occur parallel to one another and separated by only a slight thickness of 

 intervening mica schist. 



The gradation from hornblende to mica schist is always a sharp one. 

 The sheets of hornblende schist practically always occur parallel to the 

 foliation of the schist which has been shown to be parallel to the bedding 

 (PL VIII, Fig. 1). The writer did not come across a case where any 

 marked deviation from this relationship could be detected, but Dr. Charles 

 P. Berkey^^ has discovered an occurrence in mapping the geology of the 

 Tarryto\vn quadrangle where the hornblende schist cut distinctly across 

 the foliation of the mica schist, and Mr. John R. Healy^^ has observed a 

 similar case in the Catskill aqueduct under Manhattan Island. 



** Oral communication. 

 ^ Oral communication. 



