196 ANNALS NEW YORK ACADEMY OF SCIENCES 



The contacts of this gneiss with the limestone are quite sharp, but 

 nevertheless it can only be interpreted as a recrystallized interbedded 

 clastic sediment apparently of about the composition of an arkose. 



The underlying Fordham gneiss has a light gray color and a distinctly 

 gneissoid structure, being made up of a series of alternating light and 

 dark bands. On the hill Just east of the Harlem River in this vicinity, it 

 is very intricately folded and contorted. Under the microscope (PI. XII, 

 Fig. 2), it is seen to be made up largely of feldspar, quartz and mica. 

 The feldspar is mostly microcline, although some orthoclase and plagio- 

 clase are also present. The mica is for the most part a deep brown biotite 

 with some muscovite. A little sericite occurs as an alteration product 

 derived from the feldspar. Cataclastic structure is well developed. The 

 feldspar and quartz grains are, therefore, not uniform in size and are 

 usually elongated parallel to the foliation. 



The difference in structure between this gneiss and the interbedded one 

 is probably due to the fact that the limestone on either side of the inter- 

 bedded gneiss protected it from the intense crushing effect of the forces 

 accompanying the dynamic metamorphism which developed the cata- 

 clastic structure in the underlying gneiss and, therefore, only simple 

 recrystallization occurred. 



The same relation between the underlying gneiss and the limestone was 

 shown in a similar tunnel through these formations near the lower end of 

 Manhattan Island and also in the excavations for the dam at Kensico.^ 



The Inwood limestone is succeeded by the Manhattan schist. This 

 consists essentially of a coarse, quartz-mica-feldspar schist which repre- 

 sents a recrystallized sedimentary rock of more or less argillaceous com- 

 position. As would be expected in such a formation, there is considerable 

 variation from place to place and even in the same outcrop. At the con- 

 tact, the limestone frequently grades into the schist. The beds of schist 

 are also interbedded with the limestone and vice versa. Associated with 

 the mica schist are certain other types of rock, some of which are schists, 

 others gneisses, while still others are massive. They are undoubtedly of 

 igneous origin. In composition, they range from very siliceous to very 

 basic types. Their relation to the schist is such that it appears quite evi- 

 dent that they were intruded into it in part previous to, in part during, 

 and in part after the period of metamorphism. It is to a description of 

 the Manhattan schist and its associated igneous rocks that this paper is 

 mainly devoted. 



All of these formations have undergone intense metamorphism and 

 have been folded into a series of steep anticlines and synclines, which are 



• Oral communication b.v Dr. Charles P. Berkey. 



