352 PROCEEDINGS OF THE AMERICAN ACADEMY. 



lakes need not be borne in mind, as their deposits will be discussed 

 further on. The present paragraph deals only with the deposits of 

 large and comparatively deep lakes. Such deposits are of two kinds, 

 marginal and central. The marginal deposits may be relatively coarse, 

 but as such they can extend but little distance from the shore line : the 

 marginal strata of subaerial origin should be more nearly horizontal 

 than those of subaqueous origin. The central deposits are shown by 

 studies of existing lakes to be of very fine texture, such as clays, marls, 

 or very fine sands. The stratification of these fine sediments must be 

 veiy even, with few variations in texture or composition. The move- 

 ments of the waters of large lakes, either in waves or currents, do not 

 suffice to sweep pebbles out to deep water ; hence conglomerates and 

 pebbly sandstones with inclined and cross-bedded layei's must be limited 

 to a narrow belt around the lake margin. It can hardly be imagined 

 that the sediments deposited on the floor of a large lake should contain 

 frequent alternations of finer and coarser beds, such as clays and sands ; 

 but it may be inferred that the gradual filling of such a lake would allow 

 the encroachment of the later marginal beds upon the earlier central 

 ones ; and thus a relatively thin cover of coarse and variable deposits 

 might come to overlie a heavy body of fine and uniform deposits. 



If deformation or climatic change should cause repeated variations in 

 the area and depth of a large lake, a complicated series of lacustrine, 

 fluviatile, and subaerial deposits might result; but this will not be 

 further discussed for the present, as the reports referred to above sel- 

 dom explicitly recognize variations of area and depth, excepting such as 

 occur at the times of separation of successive formations to which differ- 

 ent names are given. The manner of mention of the water body in 

 which each formation was believed to have been deposited implies 

 clearly enough that it was thought to be a single, large, continuous 

 lake. 



If we now turn to the deposits of large shallow lakes, they are found 

 to be more variable in composition, texture, and structure ; for the 

 waves may stir up the materials of the bottom and the currents may 

 shift the materials from place to place ; but conglomerates need not 

 be expected to occur among them except close to the shore line. Large 

 shallow lakes cannot, however, be of great importance geologically, for 

 they must be rare and short-lived : rare, because their production de- 

 pends on the accidental concurrence of unrelated conditions ; namely, 

 the crustal deformation of aflat region by a small and nearly uniform 

 amount over a large area ; short-lived, because their preservation de- 



