DAVIS. — ROCKY MOUNTAIN TERTIARIES. 371 



of the world to-day, how can the products of similar conditions in the 

 past be recognized ? In both cases, the deposits are stratified ; in both* 

 cases, the deposits may include fine as well as coarse materials ; in both 

 cases, the area of deposition may be large as well as small ; in both cases, 

 the thickness of deposits may be great as well as light; in both cases, 

 the strata may bear ripple-marks, mud-cracks, cross-bedding, and other 

 indications of small and variable water depth. With all these similarities, 

 it would not be remarkable if a lake deposit were sometimes called a 

 river deposit, or if a river deposit were mistaken for a lake deposit ; for 

 the safe discrimination of the two classes of deposits must depend on 

 their differences, not on their resemblances. While the marginal sedi- 

 ments of a lake may be coarse, the body of the central sediments must 

 be fine and uniform. The marginal parts of a fluviatile deposit may also 

 be coarser than the forward parts, but the latter may be characterized 

 by frequent variations of texture and structure, and occasionally by 

 filled channels and lateral uncomformities. The oi'igin of the western 

 fresh-water Tertiary formations should be considered with all these 

 items in mind, and with an equal hosj)itality to the fluviatile and to the 

 lacustrine theory. 



In whatever way the discussion on this subject may end, it may be 

 noted a considerable body of geological opinion will follow its decision. 

 The lacustrine origin of the fresh-water Tertiary formations has had an 

 acceptance so general and undisputed that it has for some years held a 

 place in the geological history of the Eocky mountain region as an 

 established doctrine. Many examples of this might be given. Button 

 writes : — "I know of no more impressive and surprising fact in western 

 geology than the well attested observation that most of that [High 

 Plateau] area has been covered by fresh-water lakes. . . . The marvel 

 is not in the fact that here and there we find the vestiges of a great lake, 

 but that we find those vestiges everywhere. The whole region, with 

 the exception of the mountain platforms and pre-existing mainlands, has 

 passed through this lacustrine stage" (Monogr. II., U. S. G. S., 216). 

 The occurrence of numerous Tertiary lakes is made the basis of infer- 

 ences concerning Tertiary climate by the same author : — '•' We know 

 that the Miocene climate of the west was moist and subtropical. This is 

 indicated by the great extent of fresh-water lakes in some joortions of 

 the west, their abundant vegetable remains, and the exuberance of land 

 life " (Ibid., 223) ; but fluviatile plains do not imply a moist climate. 

 A low stand of the western part of the continent during part of Tertiary 

 time as inferred by the same geologist from the long endurance of 



