Geology and Mineralogy. 69 



that these shales mark the close of the Jurassic is here tempo- 

 rarily accepted. The reviewers believe, however, that the 

 Jurassic also includes the overlying series of sandstones and 

 shales (the " Lakota ") which are assigned to the Cretaceous by 

 Mr. Darton. Some variation in the deposits are to be expected, 

 but there is no decided physical break and the presence of typ- 

 ically Jurassic fossils, Stegosaurus, Camptosaurus, etc., collected 

 by Wieland, cannot be overlooked. The testimony of the 

 fossil plants opposes nothing if it does not support this view. 

 At the top of the " Lakota " is a very significant layer of lime- 

 stone — the " Minnewaste." That the Jurassic closed here, and 

 that subsequently markedly new conditions of plant and animal 

 life prevailed, seems probable. The overlying series of soft rocks, 

 the "Fuson" formations with scattering dicotyls, may, there- 

 fore, better represent Lower Cretaceous, followed by the Upper 

 Cretaceous Dakota sandstone. 



As regards the Tertiary sediments, the report contains much 

 that is new, interesting and important. It is the first serious 

 attempt to give in detail the distributional relations of these beds 

 to the original dome, and it is a matter of some surprise to learn 

 the extent to which the dome was originally covered by them. 

 Mr. Darton remarks (p. 558) : "They are found up to high alti- 

 tudes in the vicinity of Lead at an elevation of over 5,200 feet 

 and on the north end of the Bear Lodge Mountains more than a 

 thousand feet higher." A Post-Oligocene uplift amounting to 

 several thousand feet, and according to Mr. Darton equal to the 

 present upslope of the plains, has taken place. 



Many of the facts brought out in the present work concerning 

 these deposits cannot fail to be of great interest at this particular 

 time on account of the discussion now in progress regarding the 

 manner of deposition not only of these beds, but practically the 

 whole of the western fresh-water Tertiaries. Mr. Darton does 

 not hesitate to express his belief that they represent true lacus- 

 trine deposits as opposed to the flood-plain and aeolian hypotheses. 

 As regards the eastern barrier of the waters of the White River 

 lake, about which there seems to be so much doubt, Mr. Darton 

 says (p. 559) : "The eastern margin of this lake has not been 

 traced for any distance, but there is no great difficulty in seeing 

 that it consisted of low hills of Pierre shale and Fox Hills beds 

 against w r hich the Tertiary formations now abut to the east." 

 It is to be regretted that the author has not seen fit to enter some- 

 what more fully into a discussion of the evidences which led him 

 to conclude that these beds are of purely lacustrine origin. 



The facts attending the deposition of the mammalian fossils in 

 these beds are briefly considered. The attempt to introduce new 

 names for the old, well-known and well-established subdivisions 

 of the Tertiary deposits is open to criticism. These subdivisions 

 have been made upon the very best and most conclusive faunal 

 evidences, appropriate and widely accepted names have been 

 applied to them and there seems to be no reasons for changing 



