December 4, 1896.] 



SCIENCE. 



807 



sic strata in the West, especially the varie- 

 gated fresh- water deposits, are so striking 

 that, once seen, they will not soon be for- 

 gotten. As these physical characters may 

 be used as one means of readily identifying 

 this horizon, I have brought here, besides 

 the colored drawing of the Como section in 

 Wyoming, two others illustrating sections 

 in Colorado. One is from Morrison, near 

 Denver, and the other one hundred miles 

 farther south, near Canon City, both repre- 

 senting, in the Atlantosaurus beds, locali- 

 ties famous for the vertebrate fossils they 

 have furnished. I know of no other geo- 

 logical horizon in the West marked by such 

 striking and characteristic physical features. 



THE PLEUROCOeLUS BEDS. 



In the East, the strata most resembling 

 the Atlantosaurus beds in physical charac- 

 ters are the Potomac clays and sands so con- 

 spicuous between Washington and Balti- 

 more, and known to extend, also, both to the 

 north and south. Although fifteen hun- 

 dred miles to the eastward, these Maryland 

 strata so strongly recalled those I had ex- 

 plored at the base of the Rocky Mountains, 

 I felt reasonably sure, even before I had 

 examined them, that this series would turn 

 out to be essentially the same age as the 

 Atlantosaurus beds of the West. This 

 proved to be the case. Although the Poto- 

 mac beds have been generally regarded as 

 Cretaceous, I can now safely say that the 

 vertebrate fossils I have secured from them, 

 especially the Sauropoda, demonstrate their 

 Jurassic age beyond reasonable doubt. I 

 stated this conclusion in my first descrip- 

 tion of Potomac fossils, and it is now fally 

 confirmed by more recent discoveries.* 



The fact that the Sauropoda of the Poto- 

 mac beds are all of diminutive size, in com- 

 parison with the western forms, is a point 

 of some importance in estimating the age 



* American Journal of Science, Vol. XXXV., p. 90, 

 1888. See also, Sixteenth Annual Eeport U. S. Geol. 

 Survey, Part I., p. 183, 1896. 



of the strata that contain them. It is a rule 

 almost without exception, that the earlier 

 members of an order of ancient vertebrate 

 animals are small, while the last survivors 

 before extinction are the largest. The gigan- 

 tic forms of every such group left no suc- 

 cessors. Hence the small Pleurocoelidce of 

 the East may possibly be the ancestors of 

 of the huge western Atlantosauridce, but can 

 hardly be their descendants. The other 

 vertebrate fossils from the Potomac of Mary- 

 land, although fragmentary, all appear to 

 be Jurassic in type. 



It cannot, of course, be positively asserted 

 at present that the entire series now known 

 as Potomac is all Jurassic, or represents the 

 whole Jurassic. The Lias appears to be 

 wanting, and some of the upper strata may 

 possibly prove to belong to the Dakota. 



The latter formation in the West often 

 lies apparently conformably on the Atlanto- 

 saurus beds, and besides its many fossil 

 plants contains fragments of bones, but 

 these may have washed out of the Jurassic 

 clays below. Footprints resembling those of 

 birds have also been found. 



THE POTOMAC FORMATION. 



The Maryland Potomac, as we know it 

 to-day, is the keystone to the arch. If this 

 is Jurassic, as now seems certain, it is a 

 fair conclusion that the same series of de- 

 posits, north and south, are essentially of 

 the same age. The only region along this 

 line of a thousand miles or more where a 

 systematic search for vertebrate fossils has 

 been made is in Maryland, and here a 

 rich fauna has been found. Doubtless in 

 many outcrops of this formation, animal 

 remains may be rare or absent, as they ap- 

 pear to be in the Triassic below, but verte- 

 brate life we know was abundant during 

 the Jurassic, and characteristic remains 

 will sooner or later come to light. 



Taking, then, the Potomac formation as 

 it is developed in Maryland as an eastern 



