12 ANNALS NEW YORK ACADEMY OF SCIENCES 



the limits of the township, but the work of Dana and the later and more 

 detailed investigations of George H. Williams have served to give the 

 series a rather wide repute under this name. This area is between 25 and 

 30 square miles in extent, as will be seen from the map. The same rocks 

 are found also in a small patch, less than a quarter of a square mile, on 

 Stony Point, on the west side ef the Hudson opposite the main develop- 

 ment; these have been rather carefully mapped by Dana. 2 James F. 

 Kemp has also described an extension of the series at Eosetown, N. J., 

 where several of the types have been found ; and Wm. H. Hobbs has dis- 

 covered two large and well developed patches in western Connecticut. 

 The present paper will be confined to the type development of the series 

 around Peekskill, which exhibits the greatest complexity of differentiation. 



The Cortlandt Series has hitherto, according to Williams's description, 

 been thought of as a confused aggregrate of basic rocks, in which the 

 norite type predominated. As a matter of fact, although the norites are 

 the most prominent type, examples of nearly every group from pegmatite 

 to peridotite have been found, with local developments of very peculiar 

 and abnormal rocks. It is thus much more complete than was formerly 

 supposed and would merit a thorough study from the standpoint of mag- 

 matic differentiation. The rocks never show a distinct gneissoid struc- 

 ture; they are massive and unsheared, although there are several excep- 

 tions to this which will be considered later. The series is surrounded on 

 every side by the Manhattan schist, inclusion^ of which are sometimes 

 found in it but which are easily distinguishable on account of their schis- 

 tose structure. Inwood limestone, underlying the schist, outcrops along 

 the river bank from Verplanck Point north to Lent's Cove, and the inclu- 

 sions of limestone are also found in the igneous rock, but the main areas 

 are usually separated from them by the schist. Still other basic gneissoid 

 inclusions are found which resemble the gneisses of the Highlands to the 

 north. The whole Coitlandt Series, therefore, is a very complex and 

 intricate mass and presents a petrographical problem of rare interest. 



There is no lack of outcrops in the district, but it is seldom that the 

 rocks can be distinguished in the field ; all of them except the granites are 

 dark pink or gray, and while the writer after some practice became able to 

 identify typical specimens, the microscope could alone be relied on. It 

 was deemed wise, therefore, to conduct the field work in a somewhat 

 unusually careful manner; from 25 to 30 specimens to the square mile 

 were collected in the areal work alone, and ten of these on the average 

 were sectioned. Notwithstanding this care, there is, of course, consid- 



"Am. Jour. Sci., (3), XXII, 112. 1881. 



