GOO PBOCEEDINGS OF THE WASHINGTON MEETING 



the floors of large and deep lakes. The testimony is universal that such deposits 

 are of fine texture, and from this fact of observation it is reasonably inferred that 

 deep lake deposits must be of even stratification and of persistently uniform com- 

 position, as long as the lake remains large and deep. The marginal deposits may 

 be of tine or coarse texture, according to the nature of the shores and to the size 

 and contents of the rivers that drain them. In time these coarser deposits will 

 invade the lake basin and overlap the finer deposits of the shoaling bottom. If a 

 large shallow lake is next considered, it may be imagined that the winds could 

 produce waves and currents strong enough to produce cross-bedding and other 

 irregular structures in sandy and pebbly layers, and hence that such layers might 

 alternate with others of finer texture ; but the shallower the lake, the more rapid 

 the invasion of its shores by marginal deposits, the upper part of which will be of 

 subaerial, not lacustrine, deposition ; the shallower the lake, the more likely its 

 extinction by evaporation or by erosion of the outlet. When the lake is destroyed, 

 deposition might continue under the action of aggrading streams and rivers, and 

 thus the way is led to the consideration of fluviatile plains, many examples of 

 which, much larger in area than any existing shallow lakes, are now open to ob- 

 servation. The emphasis usually given to the destructive activities of rivers should 

 not prevent the due examination of their constructive work, so abundantly ex- 

 hibited on great river-made plains. Observation on such plains shows the capacity 

 of rivers to form deposits of variable composition, texture and structure near the 

 border of the plains, and of fine texture and comparatively even structure further 

 forward along the course of the streams. Thus the essential characteristics of lacus- 

 trine and fluviatile deposits may be generalized in form appropriate for theoretical 

 discussion. 



Choice of the Successful Theory 



The consequences of the theories of lacustrine and fluviatile deposition should 

 be confronted with the assembled and generalized facts that have been determined 

 by observation of the western Tertiaries, in order to determine how the latter are 

 best explained. It will then perhaps be possible to decide whether the Tertiary 

 epochs of Rocky Mountain history should be pictured with broad and level sheets 

 of blue water stretching between distant ranges, whether extensive gently sloping 

 plains of gravel, sand, and clay should occupy most of the intermont basins, or 

 wdiether some combination of these unlike conditions best meets the conditions of 

 nature. 



Without desiring to announce any special proportion in which the rival explana- 

 tions should be associated, and without attempting to conceal my individual lean- 

 ing toward a fluviatile or other subaerial origin for many of the formations that 

 are ordinarily described as lacustrine, it is desirable to make mention of certain 

 statements in various reports which suggest that their writers had certain qualifi- 

 cations of the purety lacustrine theory in mind. It is true that the chief emphasis 

 is given to "large lakes," and that the sediments are repeatedly said to be lake- 

 bottom formations, but the lake waters are sometimes described as shallow, and 

 alternations of shallow water and low land are given brief mention in one report, 

 which, however, elsewhere refers to the seat of deposition as " the sea" or " the 

 lake." The lake waters are described by some as having sometimes had move- 

 ment enough to distribute sands and pebbles. The movement is once characterized 



