Notices of Memoirs — Prof. 0. C. Marsh — Age of Fossils. 565 



A and B the velocity of sliding is Vj, whereas between B and C 

 it is nil. The stream lines Vj to Vg show roughly the velocities 

 along various planes in the glacier. It is clear that the ice before 

 reaching B must be greatly compressed in the line of motion, and 

 must eventually spread over the adherent portions, carrying the 

 boulders, etc., up into the mass of ice as shown in the Figure. 



That the ice may freeze firmly to the floor upon which it rests and 

 drag up and carry along with it the rocky masses thus displaced, 

 I have shown is a reasonable deduction on theoretical grounds, and 

 it is interesting to find that the adhesion of the glacier to its floor in 

 places will also account for the incorporation, in its lower portions, of 

 boulders, etc. 



ZSrOTIGSS OIP IMIZEDV-dlOIIRS. 



I. — The Comparative Value of Different Kinds of Fossils in 

 BETERMiNiNG GEOLOGICAL Age.^ By Professor 0. C. Marsh, 

 Ph.D., LL.D. 



"ORE than twenty years ago, my attention was called to the 

 subject of the difference between the value of fossil Plants, 

 Invertebrates, and Vertebrates, as evidence of the geological age of 

 the strata in which they were preserved. On the comparative value 

 of these different groups of fossils then depended the solution of 

 some grave problems in the geology of the Rocky Mountains. 

 1 therefore began a systematic investigation of the subject, arid gave 

 the results in an address befoi'e the American Association for the 

 Advancement of Science in 1877.^ I stated the case as follows : — 



"The boundary-line between the Cretaceous and Tertiary in the 

 region of the Rocky Mountains has been much in dispute during the 

 last few years, mainly in consequence of the uncertain geological 

 bearings of the fossil plants found near this horizon. The 

 accompanying invertebrate fossils have thrown little light on the 

 question, which is essentially whether the great Lignite series of the 

 AVest is uppermost Cretaceous or lowest Eocene. The evidence of 

 the numerous vertebrate remains is, in my judgment, decisive, and 

 in favour of the former view. 



1 Abstract of commumcation made to Section C, British Association for the 

 Advancement of Science, Bristol Meeting, September 9, 1898. 



* American Journal of Science, vol. xiv (November, 1877), pp. 338-378. 



