346, STUDIES FOR STUDENTS 



early in the eighteenth century the Brahmapootra joined the Ganges 

 some 60 miles above the present junction, and that for a century later 

 the Brahmapootra probably deposited nearly all its load of sediment 

 in filling up the shallow back-water lakes across which its new course 

 was taken. These lakes and swamps, having been bridged over by 

 the date cf writing, Ferguson was of the opinion that the delta face 

 would be built outward with comparative rapidity in the place where 

 it was then deficient.' 



Thus the ratio of land and marine deposition is variable even on 

 the same delta, with the season and in longer cycles. From various 

 descriptions, however, the present writer would, as his impression, 

 estimate that possibly in the case of large rivers from 30 to 50 per 

 cent, of the material reaching the sea is sufficiently fine to be borne 

 beyond the steep delta front, deposited as widespread blue muds, and 

 forming the bottomset beds. These are largely spread at the present 

 time beyond the limits of the continental platforms, and, where 

 deposited at equal depths during preceding periods, have not entered 

 to that proportion into the structure of the present continental sur- 

 faces, but still largely he beneath the sea. Of the remaining 70 to 

 50 per cent., possibly from two-thirds to three-fourths has been 

 deposited beneath the water, and from one-third to one-fourth as 

 topset beds laid down upon the land. It is unnecessary to repeat 

 that no value must be attached to these estimates. At any one time, 

 however, the area of the land-formed portion is a much greater ratio 

 the discrepancy between area and volume being due to the fact that 

 the subaerial beds are thinner, and a given thickness requires a longer 

 time to form. 



In closing the topic, the problem of the unfossiliferous nature of 

 many ancient deposits apparently well suited for the preservation of 

 fossils may be mentioned. 



It may be questioned as to why, if certain formations were deposited 

 largely upon land surfaces, those surfaces were not overgrown by 

 vegetation, preserved as carbonaceous deposits. To this it may be 

 answered that other factors, such as climate, enter into such a problem, 

 and that it may equally well be asked : How is it, if these strata belong 

 to the littoral or the shallow sea, that fossils common to those zones 



I Loc. cit., pp. 332, 333. 



