GEOLOGY OF THE NEW YORK CITY AQUEDUCT 53 



different point (Cat hill), widely separated by other types from 

 the Storm King locality, and in rather large development, is worthy 

 of separate note. It is cut, of course, in the long tunnel through 

 Cat hill. 



(lo) The Cortlandt series of gabbro-diorites occupies an area 

 of about 20 scjuare miles between Peekskill and the Croton river, 

 nearly all on the east side of the Hudson. It includes a very com- 

 plete range of coarse grained, massive, igneous rocks from soda 

 granites, grano-diorites and quartz-diorites to true diorites, norites, 

 gabbros, pyroxenites, and peridotites. They doubtless represent 

 stages or portions in the dift'erentiation of a magma. The inter- 

 relations are only partially determinable, and the petrographic dis- 

 tinctions in detail are not useful here. The area occupied by the 

 Cortlandt series has an uneven hilly surface with no structural 

 trend, and makes the most striking contrast to the ridge and longi- 

 tudinal valley structure of the rest of the region of the crystallines. 



(ii) The Peekskill granite, a white, or pink massive, very coarse 

 grained, soda granite, occupying approximately 4 square miles im- 

 mediately north of the Cortlandt area 2 miles east of Peekskill, 

 is believed to be genetically related to the Cortlandt series. The 

 evidence in favor of such a relationship has been gathered in the 

 prosecution of this work and has not been published. But it may 

 be said that the textures, structure, age, relationship to older crys- 

 tallines, interrelations with the Cortlandt series, consanguinity of 

 mineralogy, and composition all point toward the above relation- 

 ship. In essential relations, therefore, it is the acid extreme of the 

 Cortlandt series. Its economic features, however, are of sufficient 

 importance and its easy differentiation from the regular Cortlandt 

 types require that it should have separate treatment. 



(12) The Rravenswood grano-diorite occurs chiefly in Brooklyn. 

 It is a slightly foliated mass intrusive in the Fordham gneiss and 

 is doubtless connected in origin with the sources of many of the 

 hornblendic intrusive bands in the Fordham and Manhattan forma- 

 tions in the district. It covers a known area of about 5 or 6 square 

 miles and may be more extensive. The rock is suitable for struc- 

 tural material and has required consideration in the study of " Dis- 

 tributary conduits " [^see pt 2 East River section] . 



(13) Pegmatites. The pegmatites and peginatitic granophyric 

 masses of all kinds are of almost universal distribution in the 

 foliated crystallines. They vary from quartz bunches or stringers 

 to pegmatitic lenses and irregular masses, and to definite granitic 



