August 5, 1898.] 



SCIENCE. 



151. 



in strata of undoubted Jurassic age. The 

 same genus is well represented in the Po- 

 tomac deposits of Maryland, and has been 

 found, also, in the Atlantosaurus beds of 

 Wyoming, thus offering, with the asso- 

 ciated fossils, strong testimony that the 

 American and European localities are in 

 the same general horizon of the upper Jur- 

 assic."* 



Had Mr. Gilbert been familiar with the 

 subject discussed in his review he would 

 have known that, so far as present evidence 

 goes, there are other geuera of Dinosaurs 

 common to Europe and America, found in 

 apparently the same Jurassic horizon, and 

 that this is true also of various other rep- 

 tiles and of fishes. More important still is 

 the correspondence between the genera of 

 Jurassic mammals of the two continents, 



in question are in favor of their Jurassic 

 age, and the Potomac strata do not pass by 

 insensible gradation into the marine Cre- 

 taceous above. Although the two are ap- 

 parently conformable, the passage from one 

 to the other was a change from fresh- water 

 to marine deposits, which in itself implies 

 a break that may represent a long period 

 of time, perhaps the entire Lower Creta- 

 ceous. This break was clearly indicated in 

 the geological section tliat I gave in my 

 paper (figure 2), and to make this point 

 clear the same section is here repeated. 



This typical section represents the suc- 

 cessive Mesozoic and more recent forma- 

 tions, from New Brunswick, New Jersey, on 

 a line southeast, through Lower Squankum 

 to the Atlantic. The distance indicated is 

 about forty miles. 



a 1) c d 



C4eological Section in New Jersey. 

 a, Triassic ; 6, Jurassic ; e, Cretaceous ; d, Tertiary ; T, title level. 



which in itself is suflicient to demonstrate 

 that they belong in essentially the same 

 horizon. 



The last point Mr. Gilbert raises in his 

 review is a geological one, and even here 

 he has missed the mark. His words are as 

 follows : 



" The physical relations of the beds afford 

 a presumption in favor of their Cretaceous 

 age. Professor Marsh mentions that the 

 Potomac formation in New Jersey passes 

 by insensible gradation into marine Creta- 

 ceous above." 



The two statements in this quotation are, 

 in my opinion, both erroneous, and the 

 second is contrary to the idea I intended to 

 convey. The physical relations of the beds 



* American Journal of Science, Vol. IV., p. 415, 

 December, 1897. 



My explanation was as follows : 



" The change from the fresh- water plastic 

 clays of New Jersey to the marine beds con- 

 taining greensand over them proves not only 

 the breaking down of the eastern barrier 

 which protected the former strata from the 

 Atlantic, but a great subsidence also, since 

 glauconite, as a rule, is only deposited in 

 the deep, still waters of the ocean."* 



Since my paper was published, I have 

 been over part of this section several times, 

 and found clear indications of the break 

 itself. Moreover, Professor W. B. Clark, of 

 Baltimore, informs me that he finds distinct 

 unconformity between the marine Creta- 

 ceous and the underlying Potomac, along 

 the junction of these two formations, at 

 various other points further south. This 



* Science, Vol. IV., p. 812, 1896. 



