370 PROCEEDINGS OF THE AMERICAN ACADEMY. 



critical quality, but also because it is concerned with a fine-textured 

 formation which has been referred to a lacustrine origin with more 

 confidence than almost any other in the Rocky mountain region. 



16. Summary. — The object of this paper is to promote consideration 

 rather than to announce conclusions. The author's leaninsj towards the 

 explanation of parts of our western Tertiary formations as fluviatile 

 rather than as lacustrine deposits is not to be concealed, but he would 

 prefer to leave the decision of the points here mooted to those who have 

 a larger personal knowledge of the Rocky mountain region than he has. 

 Observations recorded by western explorers and geologists are in no 

 case disputed ; descriptions of observed structures are accepted as if 

 they were first-hand facts ; but the origin attributed to the observed 

 structures is not always accepted, because it is thought that other 

 interpretations than those offered by the observers are in some cases 

 to be preferred. This essay is therefore not concerned with the direct 

 observation of facts in the Rocky mountain region, but with the the-' 

 oretical discussion of recorded observations, a discussion in which any 

 one may reasonably take part, whether he has visited the region under 

 consideration or not, provided that the facts of structure are well observed 

 and described by those who have been there. 



It is in no spirit of antagonism, but simply from interest in a 

 theoretical problem that expression is here given to an opinion in a 

 matter of interpretation different from that adopted by the authors 

 whose observations are quoted; an opinion which it seems advisable to 

 express freely in order to bring the discussion of the problem to the 

 attention of geologists interested in this aspect of their science. 



The points in discussion seem to be in the main these : — Lake waters 

 may receive materials of various kinds from their shores and from the 

 rivers running into them ; and the materials thus gathered, coarse and 

 fine, will be deposited in stratified arrangement near the border and over 

 the floor of the lake. Their volume will depend on the rate, area, and 

 period of accumulation. River basins may also, under certain con- 

 ditions, receive in their lower portions more material than can be carried 

 forward and discharged into the sea ; or interior basins not containing 

 lakes may be gradually aggraded by the materials washed in by streams 

 from the enclosing highlands : here again the materials may be coarse 

 and fine ; they may have a stratified arrangement, the area of deposition 

 may be small or large ; and the volume of the deposits will depend on the 

 rate, area, and period of accumulation. 



Recognizing these possibilities, assured by observation in various parts 



