18 ANNALS NEW YORE ACADEMY OF SCIENCES 



most of the types are found in a small patch about two miles square at 

 Prospect Hill itself. The igneous rocks are included in highly metamor- 

 phosed gneiss, quartzite and schist of pre-Cambrian and early Paleozoic 

 age, so that they are probably post-Ordovician. Along the border, there is 

 generally a mosaic of block faults. The rocks may carry considerable 

 percentages of pyrrhotite and chalcopyrite, which are exploited in many 

 prospect holes known locally as "nickel mines." Hobbs describes the 

 rocks as varieties of the following types: gabbroitic. noritic, olivine- 

 hypersthene gabbroitic, dioritic, peridotitic, pyroxenitic and grano-dio- 

 ritic. Although his system of nomenclature differs somewhat from that 

 followed in the present paper, it seems that all of these types are counter- 

 parts of those found in the main area. The most salient points of differ- 

 ence are the abundance of somewhat more basic plagioclase and of chal- 

 copyrite and pyrrhotite, with the absence of emery, in the Connecticut 

 areas. Five complete and valuable analyses are given, and the differentia- 

 tion is very suggestively summarized in a series of Brogger diagrams. In 

 the smaller, Xew Fairfield, area, the rocks are all of a dioritic nature, 

 resembling in this respect the Rosetown, N. J., extension. 



From the description given, it is evident that the larger area at least 

 would warrant close study ; it is to be regretted that the boundaries of the 

 main districts, and the approximate distribution of the many interesting 

 types described, have not been more thoroughly worked out. 



Charles P. Berkey, 17 in connection with an. exhaustive study of the 

 geology of the Highlands and the region to the south, has materially 

 added to our knowledge of the Cortlandt Series. He has been the first 

 geologist to map the boundaries ; the writer has, of course, gone over the 

 same ground in connection with this present study, and the results are 

 entirely concordant. To Dr. Berkey also belongs the credit for the recog- 

 nition of the acid extreme of the series; the large granite area to the 

 northeast of the main body was overlooked by both Dana and Williams. 

 As will be shown later, there is every reason for considering this an inte- 

 gral part of the series ; and if this be so, it completes the line of differen- 

 tiation from the most acid to the most basic, being thus an important 

 consideration from a theoretical standpoint. The writer is under great 

 obligation to Dr. Berkey for this as yet unpublished information. 



This, then, constitutes the whole of the work done on the Cortlandt 

 Series. As will be seen, some of it has been superficial, some ill-directed 

 and some disproportionately minute. Kemp's work on the Rosetown 



17 "Structural and stratigraphic features of the basal gneisses of the Highlands," Bull. 

 N. Y. State Museum, 107 (Geology 12). 1907. This paper is the first in connection 

 with this study and is not concerned with the Cortlandt Series. 



