G. S. Rogers — Original Gneissoid Structure. 125 



Art. XVI. — Original Gneissoid Structure in the Cortlandt 

 Series ; by G. S. Rogers. 



In the course of a rather detailed investigation of the Cort- 

 landt Series the attention of the writer was attracted by a 

 peculiar structure occasionally occurring in these rocks, which 

 seems to be undoubtedly of an original gneissoid character. 

 By way of introduction a word of review about the Cortlandt 

 Series itself may not be out of place. The rocks cover an area 

 of about 25 square miles, and are situated just southeast of 

 Peekskill or about 35 miles north of New York City. They 

 constitute a small but rather complete igneous complex, con- 

 taining examples of all of the main varieties from granite to 

 peridotite. Although hitherto they have been thought of as a 

 wholly basic group of rocks, more careful work reveals the 

 fact that nearly a third of the whole series consists of granite, 

 syenite, and a diorite often acid enough to be called monzon- 

 ite. Pyroxenites, often chry soli tic, make up somewhat less 

 than a third, while several varieties of norite comprise most of 

 the remaining types. Trachyte, sodalite-syenite, gabbro, and 

 many dike rocks are also represented, and there are moreover 

 several contact developments of very peculiar and abnormal 

 composition. Finally, emery has been mined for the last 30 

 years, chiefly along the borders of the district. 



The Cortlandt Series is rather well known from the work 

 already done on it by two eminent geologists, Professors J. D. 

 Dana and Gr. H. "Williams. Professor Dana* described the 

 rocks in connection with his work on the limestone belts of 

 Westchester County. He directed his attention especially to 

 the origin of the rocks, giving only a brief general description 

 of their petrographic characters. He noticed, however, that 

 On Montrose Point several very different kinds of rock are 

 associated in the most intricate way, generally as successive 

 bands ; and cites one case in which norite and pyroxenite are 

 found in alternate layers of constant grain only three or four 

 inches wide. There are also other less pronounced cases ; and 

 from these phenomena, as well as from the occasional streaked 

 appearance of the rocks, Dana concludes that they were orig- 

 iually volcanic ashes or tuffs, which, on being subjected to 

 intense local metamorphism, lost most of their bedded struc- 

 ture and became pseudo-massive. In his last paper, however, 

 which is based on the revelations of the new railroad cut 



* Geological Relations of the Limestone Belts of Westchester Co., N. Y., 

 this Journal (3), xx, 194. Also, Origin of the Rock of the Cortlandt Series, 

 idem (3). xxii, 103 ; and Note on the Cortlandt and Stony Point Hornblendic 

 and Augitic Rock, idem (3), xxviii, 384. 



