ROGERS, GEOLOGY OF THE CORTLANDT SERIES 15 



this explanation, arguing that sediment may change suddenly in composi- 

 tion but that igneous flows are constant over large areas. For the source 

 of the heat, he postulates extreme local metamorphism of some kind. 



In his second paper, 7 he reviews the above arguments and proceeds to 

 the question of the origin and nature of the sediments. The rocks are so 

 very different in composition from the surrounding sedimentary rocks 

 that some peculiar local development must be postulated. There are 

 three hypotheses : ( 1 ) the material may have been contributed by the 

 Archean Highlands as detritus, or (2) as detritus supplemented by in- 

 gredients from the ocean, or (3) it may have been originally volcanic 

 ashes. While expressing no definite opinion, Dana appears to favor the 

 last theory, since successive outbursts might be very different in composi- 

 tion and yet come to rest in close juxtaposition. He suggests that the 

 volcanic source lay in the present bed of the Hudson, between Verplanck 

 and Montrose Points. 



In his third paper, 8 he describes the phenomena exposed by the (then) 

 new railroad cut through Stony Point, which he considers entirely indica- 

 tive of the true igneous nature of the more basic members of the series at 

 least. He abandons, of course, all of the above theories ; and this affords 

 bxl excellent example of the frankness, honesty and sincerity which char- 

 acterizes his life and work. 



George H. Williams, at about this time, started a minute petrographic 

 study of the various types in the district, Dana having called his attention 

 to the wealth of variation exhibited. He did not, however, publish his 

 first paper until 1886. 9 After reviewing Dana's work, he announces his 

 intention of describing the types petrographically, beginning with the 

 most basic ; then the contact metamorphism in the schist around the bor- 

 ders ; and finally of giving his theoretical conclusions and generalizations. 

 Starting with peridotite, he describes hornblende peridotite, devoting con- 

 siderable time to the schiller structure so often found in all the rocks of 

 the series, and then augite peridotite (picrite), giving an analysis of the 

 latter. He takes up each mineral, discussing its general habit and its 

 special peculiarities in these rocks, and often drawing valuable and schol- 

 arly comparisons with its occurrences abroad. 



His second paper 10 is on the norites, which, he says, are by far the most 

 abundant type in the district and cover the whole township east of the 



7 "Origin of the Rocks of the Cortlandt Series," Amer. Jour. Sci., (3), XXII, 103. 

 1881. 



8 "Note on the Cortlandt and Stony Point Hornblendic and Augitic Rock," Amer. Jour. 

 Sci., (3), XXVIII, 384. 1884. 



9 "Peridotites of the Cortlandt Series," Amer. Jour. Sci., (3), XXXI. 26. 1886. 



10 "Norites of the Cortlandt Series," Amer. Jour. Sci., (3), XXXIII, 135, 191. 1887. 



