Geology and Mineralogy. 197 



II. Geology and Mineralogy. 



1. Cycad Horizons in the Rocky Mountain Region ; by O. C. 

 Marsh. — After the article on the Jurassic Formation in this num- 

 ber of the Journal (pp. 105-115) was in print, I received some 

 information about Cycad horizons in Wyoming, that bore directly 

 on the question I discussed near the end of my paper. This 

 information was of so much interest that I added a foot-note on 

 p. 115, to place on record the important discovery by W. H. 

 Reed, of two new Cycad localities in the Jurassic of Wyoming, 

 both much farther west, and quite distinct from those already 

 known around the Black Hills. One is in the Freeze Out Hills 

 of Carbon County, and the other near the Wind River range. 



Mr. Reed has since sent me a more complete account of the 

 first of these localities, with a sketch showing the section of the 

 strata where the Cycads were found, and also measurements of 

 the successive strata exposed, from the Trias up to the so-called 

 Dakota sandstone, that caps the bluff at that point. The marine 

 Baptanodon beds here show a thickness of thirty-five feet. Above 

 these is a series of fresh-water sandstones and shales, sixty-six 

 feet in thickness, which in places contain remains of Laosaurus, a 

 typical Jurassic Dinosaur. Immediately above these the Cycads 

 occur in a narrow layer of white sandstone, and with them are 

 various fragments of bones. Xext above are fifty-five feet of 

 strata containing vertebrate fossils, apparently indicating the 

 Atlantosaurus beds. Above these are thirty feet of barren clays, 

 and over all is the sandstone regarded as Dakota. 



Mr. Reed has also sent me specimens of the Cycads found at 

 this locality. As he has had an experience of twenty years or 

 more on the Jurassic of the West, and is otherwise admirably 

 qualified to judge of such horizons, his opinion is entitled to great 

 weight, and should settle the question for this locality. 



Mr. H. F. Wells, who has carefully explored the Black Hills 

 Cycad horizon, and sent to the Yale Museum over one hundred 

 specimens of these fossils, has also, at my request, sent me a sketch 

 of a section near Blackhawk on the eastern rim of the Hills, 

 a region which I have myself examined, although not recently. 

 This section indicates that the Cycad horizon there is also in the 

 Jurassic, and not in the Dakota, and this is borne out by other 

 localities in the same vicinity. 



Prof. L. F. Ward has published sections examined by him on 

 the southwestern border of the Black Hills in 1893. He found 

 no Cycads actually in place, bat decided that the horizon in which 

 they occur is Cretaceous.* I have recently placed in his hands 

 for description all the western Cycads in the Yale Museum. Our 

 views, however, do not at present coincide as to the age of the 

 strata containing them, but the new facts which are now being 

 brought to light will, I trust, soon place this matter beyond 

 reasonable doubt. 



Yale University, July 18, 1898. 



* Journal of Geology, Vol. II, p. 250, 1894. 



