352 PROCEEDINGS OF THE AMERICAN ACADEMY. 



lakes Deed Dot 1"- bonie id mind, as their deposits will be discussed 

 farther on. The present paragraph deals only with the depos - 

 and comparatively deep lakes. Such deposits are « * t" two ki: 

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

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

 marginal strata of subaerial origin Bhould be mure nearly horizontal 

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

 studies of existing lakes to be of very line texture, Mich as clays, marls. 

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

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

 ment- of the water- of large lake-, either in wave- or currents, do not 

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

 pebbly sandstones with inclined and cross-bedded layers must be limited 

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

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

 frequent alternation- of liner ami coarser hed-, such as claj - ai d sat 

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

 the encroachment of tin' later marginal hed- upon the earlier central 

 ones; ami thus a relatively thin cover of coarse and variable depi 

 might come to overlie a heavy body of line and uniform di 



[f deformation or climatic change should cause repeated variations in 

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

 ilnviatile, and subaerial deposits might result; hut this will not In- 

 further discussed for the present, as the reports n ferred to above -■ l- 

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



ur at the times of -eparation of successive formations to which differ- 

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

 which each formation was believed to have been deposited implies 

 cli irly 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 he more variable in composition, texture, and structure: for the 

 waves may stir up the material- of the bottom and the currents may 

 shift the materials from place to place ; hut conglomerates need 

 he expected to occur among them except c]o-e to the shore line. La 

 shallow lake- cannot, however, he of great importanc gically, for 



they iiiiis| he rare and -holt lived : rare, because their production de- 

 pends on the accidental concurrence of unrelated conditions ; namely, 

 the crustal deformation of a flat region by a -mill and nearly uniform 

 amount over a lai i: Bhort-lived, hecati-e their preservation de- 



