120 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
has been bored to the depth of 481 feet. The whole mass was found 
to consist of fine sands and clays with occasional pebble beds, a bed 
of peat and remains of trees; but there was no trace of marine organ- 
isms (A. Geikie, loc. cit., p. 518). Since deltas are built forward by 
the deposits of streams and distributaries which may alter their 
courses from time to time, the bedding will tend to be that of an imbri- 
cated series of lenticular masses rather than that of a uniform series 
of layers. 
Lacustrine Deposits. Delta deposits, which are so frequent along 
lake shores, have been described above. The littoral processes of 
lakes differ from those of the oceans only in the absence of tides. It 
is natural, therefore, that the deposits formed by these processes should 
be essentially similar in both cases. The main effect of the tides upon 
the deposition of sediments is probably to sort the materials more 
thoroughly. We should expect, therefore, that lacustrine formations 
would be, on the whole, less perfectly sorted than marine accumula- 
tions. The characteristics of lake sediments have been discussed to 
some extent by Delebecque in his work entitled “Les lacs Francais,” 
Fic. 3.— Diagrammatic section illustrative of the fresh-water cycle of sedi- 
mentation: aa’, coarse gravel; bb’, finer gravel and sand; c, clay (after 
Rutot and Broeck, p. 57, 346). 
but this paper is not available at the present writing. An important 
contrast may be noted in the arrangement of beds in the normal 
fresh-water cycle compared with that in the marine cycle. In the 
latter finer beds are necessarily laid over coarser during the gradual 
transgression of the sea upon the land, so that a section normally 
shows coarse sediments at the bottom with finer beds above. Figure 
1 displays such a section with some minor interruptions. In the case 
of lakes, the sediments brought in by rivers tend to encroach upon 
and fill up their basins so that the coarser materials, which are deposited 
nearest the shore, advance lakeward with the contraction of the coast 
line and thus become superposed upon finer materials which were 
deposited when the former shores were more distant. Such a section 
would show finer materials overlaid by coarser materials, as illustrated 
in Figure 3, after Rutot and Broeck. 
