Massive Crystalline Moclis. 61 



east-and-west ridge consists of rocks related to those of Montrose 

 and Cruger's Points, and there is little doubt that it was once con- 

 nected with the Montrose region. It is the only locality of the rocks 

 yet observed on the western side of the Hudson. 



The accompanyiug map of the western portion of the town of 

 Cortland, between Peekskill and Cruger's, together with the Hudson 

 Eiver adjoining, contains the places here referred to. Its scale is an 

 inch to a mile. 



The occurrence of limestone areas in close proximity to the rocks 

 of the Cortland series is a fact of special interest, as is shown 

 beyond. These areas on the maps are those horizontally lined. 



A brief description of the prominent varieties or kinds of these 

 Cortland rocks will prepare the way for a discussion of their 

 relation to the other rocks of Westchester County. 

 A. Kinds of Eocks. 



The more prominent peculiarities in the constitution of these rocks 

 (as learned by the aid of thin slices and microscopic examination) 

 are as follows : 



1. The felspars are chiefly triclinic species, or soda-lime felspars, 

 though some orthoclase (potash-felspar) is also often present. 

 They are therefore distinctively what many would call '' plagioclase " 

 rocks. 



2. One or more of the minerals of the Amphibole group — horn- 

 blende, hypersthene, augite — are jDresent in a large part of the rocks ; 

 of these, hypersthene is the most widely distributed. Its crystals, 

 which are sometimes quite perfect, have the form of the augite 

 common in volcanic rocks except that they are not oblique ; they were 

 ascertained to be true hypersthene through optical methods by Dr. 

 G. W. Hawes. 



3. Black mica or biotite is usually present, and sometimes abun- 

 dantly, and in some of the kinds replaces wholly, or nearly so, the 

 iron-bearing amphibole minerals. 



4. Quartz is not a prominent ingredient, and in general is only 

 sparingly present. 



5. Chrysolite is a characteristic ingredient of some of the common 

 kinds. 



6. Apatite exists in microscopic and sometimes visible crystals in 

 all the varieties ; the largest crystal observed has a length of half 

 an inch and diameter of a sixteenth. Magnetite is present in grains, 

 and sometimes constitutes beds. Pyrrhotite also is disseminated 

 through most of the rocks. 



These crystalline rocks are commonly massive, that is, without 

 bedding. They are everywhere jointed, and for this reason the 

 ledges are generally piles of large and small blocks. In most 

 places they undergo easy decomposition, making a grey or iron -red 

 soil around ; and, as the joints give access to water, the outer blocks 

 in the pile have often become reduced to rounded and half-detached 

 masses. 



The rocks may be divided, for tlie convenience of the stratigrai^hic 

 discussion beyond, into (1) the non-chnjsolitic, and (2) the chrysolilic. 



