414 Eeviews — United States Geological Surveys. 



Tertiary beds, the basalts and conglomerates of volcanic origin kave 

 been subject to terrific erosion, and only a portion of their wonderful 

 magnitude is left behind. So far as my own, explorations have ex- 

 tended, the main portion of the volcanic material of the West has 

 been thrown out at a comparatively modern date. Among the Cre- 

 taceous and Coal-bearing groups are irregular interstratifi.ed beds of 

 basalt, but the great mass of trachyte, basalt, and volcanic conglome- 

 rates, seems to have been erupted since the surface attained pretty 

 nearly its present configuration. The conglomerates, although in 

 some instances reaching a thickness of 3000 feet at least, do not 

 seem to have been disturbed to any great extent since their de- 

 position. The position of the trachytes, which have overflowed the 

 mountain ranges, indicates that they could not have been very 

 ancient, perhaps not older than later Miocene or early Pliocene, 

 while the true basalts are extremely modern, approaching closely to- 

 our present era." (p. 44.) 



One of the principal objects of the Survey has been to collect 

 evidence bearing on the relations of the Cretaceous and Tertiary 

 strata to each other, for considerable difference of opinion has arisen 

 as to the true age of the beds at the summit of the former and the 

 base of the latter strata. The above strata, with the exception of 

 some Carboniferons and Permian rocks, are extensively developed in 

 Nebraska north and south of the Platte River, as shown in the 

 Eeport (No. 3). The Cretaceous rocks are divided into the Dakota,^ 

 Benton, and Niobrara groups in the lower series, and the Fort Pierre 

 and the Foxhills group in the upper series. Above these is the 

 Fort Union or Lignite group, which, with some other beds. Dr. 

 Hayden has termed " transition beds,'^ as they indicate a passage 

 from marine to lacustrine conditions. 



In Nebraska these lignite beds, with great numbers of dicotyle- 

 donous leaves, are doubtfully placed as Eocene (p. 93), and are over- 

 lain by the Wind river and White river group referred to the 

 Miocene, and the Loup river beds, with freshwater and land types, 

 to the Pliocene period. The problem of the relations of the Creta- 

 ceous and Tertiary and Transition strata, has been approached (at 

 Dr. Hayden's request) by Profs. Leidy and Cope, through the 

 extinct Vertebrate fauna, by Mr. Lesquereux through the Fossil flora, 

 and by Mr. F. B. Meek through the study of the Invertebrata. These 

 papers, printed in the above Keports (Nos. 2, 3, 4), although to some 

 extent preliminary, are full of valuable and suggestive remarks, well 

 worthy the study of the European palaeontologist, and as showing, from 

 each author's stand-point of view, the age of the formations under 

 discussion. The subject is further supplemented by the splendid 

 memoir, "On the Extinct Vertebrata of our Western Territories," by 

 Prof. J. Leidy (No. 5 of the above list), containing descriptions of 

 125 species of mammals, reptiles, and fishes, and forming the first 

 part of vol. i. of the quarto series of reports which are intended to 

 embody the more original and technical results of the Survey. 



The study of the fauna and flora of these different formations 

 yields somewhat discrepant results, for, as remarked by Professor 



