l62 



SCIENCE. 



[Vol. XIV. No. 344 



a view is untenable when all the known facts^ are considered. It 

 is at present sufficient to say that the great break between the 

 mesozoic and paleozoic of North America occurred while yet pale- 

 ozoic forms of life were far in excess of mesozoic forms ; and that 

 almost all the North American strata that have been recognized as 

 of Permian age appear to have been the result of continuous sedi- 

 mentation from the carboniferous. In short, all the hitherto rec- 

 ognized or reputed Permian of North America is far more inti- 

 mately related, both paleontologically and stratigraphically, with 

 the paleozoic than with the mesozoic. Therefore the lower delimi- 

 tation of the North American mesozoic must coincide with the 

 base of the lowermost discovered.triassic strata. 



A few hundred feet in thickness of strata, which have by com- 

 mon consent long been referred to the Jurassic, are found within 

 a large part of the middle portion of the interior region, resting 

 conformably upon the triassic strata which have already been no- 

 ticed. Where these Jurassic strata have been fully studied, espe- 

 cially in Colorado and Wyoming, they are separable into an upper 

 and a lower portion, the lower portion being of marine, and the 

 upper of fresh-water origin. The invertebrate fossils of the upper 

 portion are mostly of types that are now living, and are, therefore, 

 of no value as indicating their geological age. Those of the lower 

 portion are few, and the cephalopods only, or mainly, present such 

 characters as to suggest their Jurassic age ; and it was upon this 

 slight evidence, together with the relative position of the strata, 

 that their reference to the Jurassic was first made. 



Professor Marsh's well-known publications of the remarkable 

 dinosaurian faunas from both the upper and lower portions of the 

 strata in question have left no reasonable doubt that they are 

 really of Jurassic age. Professor Marsh refers all these strata to 

 the upper Jurassic of Europe ; and in connection with this state- 

 ment I wish to call your attention to the fact that wherever they 

 have been found in contact with the triassic strata already dis- 

 cussed, they are not only strictly conformable, but they seem to 

 have been the result of continuous sedimentation. In fact, it is 

 paleontology alone that suggests an hiatus between them. The 

 field geologist finds no evidence of it. 



The Jurassic rocks of the interior region disappear both to the 

 northward and southward, their geographical range being appar- 

 ently a little less than that of the underlying triassic beds. No 

 equivalent of the former has been found in Canada, although the 

 cretaceous Dakota group, which immediately overlies the Jurassic 

 further southward, has been recognized there. It may be re- 

 marked also that where the Jurassic is not present beneath the 

 cretaceous, the latter, especially in the eastern part of the region, 

 is often found resting directly upon the older rocks, sometimes 

 even upon the archsean. In Texas the Jurassic is also absent 

 from beneath the marine formation, which is regarded as the rep- 

 resentative of the Dakota group there, the latter resting directly 

 but uncomformably upon the Comanche beds, to be presently no- 

 ticed. 



Omitting present consideration of the isolated masses of reputed 

 Jurassic rocks in western Nevada and eastern California, this sub- 

 division of the mesozoic seems to be represented in North America 

 mainly by the slight accumulation of strata in the interior region 

 which has just been noticed. We know little or nothing of the 

 flora which existed when these strata were deposited ; their inver- 

 tebrate fossils are of little value in determining their geological age, 

 and if it were not for their dinosaurian faunas their Jurassic age 

 might well be questioned. 



The section of the cretaceous formations which prevail in the 

 central portion of the interior region, and to which I shall more 

 particularly refer in following remarks, differs materially from a 

 similar section in the southern portion, usually known as the Texas 

 section. Meek and Hayden divided the cretaceous of the central 

 portion of the region into the Dakota, Benton, Niobrara, Pierre, 

 and Fox Hills groups, the first mentioned being the earliest, and 

 the last one mentioned the latest. In Texas the cretaceous sec- 

 tion is continued much beneath the equivalent of the Dakota group 

 there. These lower Texan strata constitute the important marine 

 formation now known as the Comanche beds, the molluscan fauna 

 of which gives peculiar paleontological character to the Texas sec- 

 tion. Above the Comanche beds there is a series of formations 



that are understood to respectively represent all the more northern 

 formations which have just been mentioned. 



After due consideration of all the known facts, some of which 

 are of recent acquisition, there seems to be no room for reasonable 

 doubt that the marine cretaceous deposits of the interior region 

 which are later than the Dakota group are, as a whole, not only 

 equivalent with the marine cretaceous deposits of the Atlantic and 

 Gulf coast region, but that they were all originally continuous 

 through the whole of that great geographical extent. These for- 

 mations are too well known to need present characterization ; and 

 they are now known to constitute the most extensive and definite 

 taxonomic horizon that has been recognized among the mesozoic 

 formations of this continent. Furthermore, the marine molluscan 

 fauna of these strata is of such a character as to leave little room 

 for doubt that they represent homotaxially the Senonian, and per- 

 haps a part of the Danian, of Europe. The difficulty, however, of 

 accurately correlating the cretaceous formations of this continent 

 with those of Europe is very great, as has, for example, lately been 

 indicated by Professor Rosmer's reference of certain fossils of the 

 Comanche beds to the upper Turonian. These beds lie wholly 

 and unconformably beneath the horizon of the Dakota group, 

 which is itself not probably newer than the Cenomanian. 



Before proceeding to a consideration of the Laramie group, it is 

 proper to say that the presence in British America of the Kootanie 

 formation beneath the Dakota group, and that of the Comanche 

 beds beneath the equivalent of the latter in Texas, shows that 

 there is really an hiatus between the Dakota and the Jurassic in 

 the interior region, although their conformity is so complete that it 

 has never been detected by field observation. If a similar hiatus 

 exists between the Jurassic and triassic in the same region, we 

 have also no stratigraphical evidence of it. 



The Laramie is in many respects one of the most remarkable of 

 the North American formations. It is found occupying large por- 

 tions of the interior region from the State of Nuevo Leon in Mex- 

 ico to beyond 52° north latitude. It reaches a maximum thickness 

 of nearly four thousand feet in Colorado, and more than that in 

 British America. It is not only everywhere conformable upon the 

 Fox Hills group, but wherever the junction between them has 

 been seen, sedimentation from the older to the later formation 

 appears to have been continuous. 



In all its great geographical extent the Laramie group has never 

 been found to contain any animal remains similar to those which 

 inhabit the open sea only. A considerable proportion of its inver- 

 tebrates are like those which are now denizens of brackish waters, 

 and a still greater proportion are fresh-water forms. It is mainly 

 upon this abrupt change from a marine to a brackish and fresh 

 water character of the molluscan fossils, and not upon strati- 

 graphical difference, that we rely to determine the lower limit of 

 the Laramie formation. 



The labors of Dr. G. M. Dawson and Mr. Whiteaves, and their 

 associates in the Canadian Survey, have shown that conditions 

 similar to those which gave character to the Laramie formation ex- 

 isted in a large part of the northern interior region long before the 

 close of the Fox Hills epoch, and that they were probably continued 

 into the Laramie epoch. But time will not permit me now to dis- 

 cuss this interesting question. 



Besides the invertebrate fauna which has just been referred to, a 

 few insect remains, a rich flora and a somewhat extensive and va- 

 ried vertebrate fauna have been obtained from the Laramie forma- 

 tion. None of the molluscan remains, so far as I can judge, pos- 

 sess characters which any similar forms might not have possessed 

 at any time from the middle cretaceous to the eocene inclusive ; 

 and a large part of them differ from living forms only as species. 



Siinilar remarks may be properly made concerning the plant re- 

 mains of the Laramie formation. Professor Ward has shown that 

 of the one hundred and twelve genera of plants which have been 

 discovered in the Laramie, thirty-eight of the genera and five of the 

 species are common to the Dakota Group ; eighty-five of the gen- 

 era are living and twenty-seven are extinct. These extinct genera 

 are all so nearly allied to living genera respectively that it is diffi-. 

 cult to separate them. Furthermore, not less than three species 

 from the upper strata of the Laramie have been identified with liv- 

 ing species. 



