438 Williams — Gabbros and Diorites of the Cortlandt Series. 



cept the great hiatus which probably exists between the 

 Carboniferous, and the Uinta Sandstone. The sedimenta- 

 tion also seems to have been continuous from the upper- 

 most of the marine Cretaceous formations into the Laramie, 

 although the faunas of these respective groups are widely dif- 

 ferent. Consequently field geologists have always experienced 

 great difficulty, in the frequent absence of distinguishing fos- 

 sils, in separating that marine Cretaceous formation from the 

 Laramie ; and they have therefore been disposed to regard the 

 latter as a Cretaceous formation. 



While I still believe that at least the upper strata of- the 

 Laramie Group represents a gradual transition from the Cre- 

 taceous to the Tertiary period, the facts which have been pre- 

 sented in the preceding paragraphs certainly constitute strong 

 presumptive evidence of the Cretaceous age of the greater 

 part of it. Judging from my own investigations, it is regarded 

 as impossible to draw either a paleontological or a stratigraph- 

 ical dividing line between the Cretaceous and Tertiary por- 

 tions of the Laramie Group. Therefore the established cus- 

 tom of geologists in formulating a scheme of classification of 

 the formations, seems to require that the whole group should 

 be classed either as Cretaceous or Tertiary. It is not only con- 

 ceivable, but it is natural to suppose, that a transitional forma- 

 tion might possess characteristics which, so far as evidence of 

 age is concerned, would be nearly equally balanced between 

 two periods. I believe the Chico-Tejon series of California, 

 for example, actually presents just such a case. The evidence, 

 as a whole in the case of the Laramie however does not appear 

 to be so well balanced, and in my future writings I shall prob- 

 ably class the Laramie as a Cretaceous formation ; although I 

 shall regard this practice as little more than a matter of con- 

 ventional convenience. 



Akt. XXXVIII.— The Gabbros and Diorites of the " Cort- 

 landt Series " on the Hudson Hiver near TeeksMll, N. Y.' } 

 by George H. Williams. 



In two former papers* I have described two types — perido- 

 tites and norites — which form members of that complex group 

 of massive rocks occurring in the northwestern corner of West- 

 chester County, ]ST. Y., and designated by Prof. J. D. Dana as 

 the " Cortlandt Series." The area occupied by these rocks — 

 about twenty-five square miles in extent and nearly coincident 



* This Journal, III, xxxi, Jan. 1886, p. 26; ib., xxxiii, Feb. and March, 1887, 

 p. 135 and p. 191. 



