490 THE JOURNAL OF GEOLOGY. 



peared clear, and we could only trace one part of solid matter 

 held in suspension by 25,925 of water. Nothing could more 

 forcibly illustrate the formation of deltas. The river retains 

 matter held in suspension by its water within its ordinary chan- 

 nel as long as its velocity is maintained ; as soon as it enters a lake 

 or an estuary checking regular currents, the matter held in sus- 

 pension is dropped." 



That is to say, in flowing 142 miles in its navigable channel 

 and through its delta the river dropped about 1 5 per cent, 

 of the load which it bore at Salto ; and beyond the delta in 

 still water it dropped 48 per cent, more; leaving it but 37 

 per cent, of the original load to be carried past Higueritas to the 

 estuary of the La Plata. Or stating the proportions in terms of 

 the sediment brought through the delta to the head of the lake, 

 57 per cent, was deposited and 43 per cent, escaped. It would be 

 desirable to determine in what ratio the deposit is made in the 

 upper and lower reaches of the lake, but Revy gives no data 

 between Fray Bentos and Higueritas. He states however that the 

 lake is without islands, although it is shallow with the exception 

 of a deep channel half a mile wide ; but just above Fray Bentos 

 islands indicate the present front of the delta. The occurrence 

 of these advance elements of the delta only in a limited distance 

 indicates that the bulk of deposition is on the delta's front, and 

 that the sediment which passes beyond is that which the slower 

 current of the lake can hold in suspension. 



The deposits of the extinct lakes Bonneville and Lahontan 

 have been fully described hy Gilbert and Russell, but the lake 

 beds of the west still present rich fields for study of deposition 

 under simple conditions in fresh and salt water. 



(r) Alterations of Current. — When a land-locked water body 

 is open to the ocean it is subject to influx and reflux of tides, but 

 the rivers pouring into it may possess volume sufficient to exclude 

 salt water ; it is then a fresh-water estuary, which receives the 

 sediments as well as the waters of its tributaries. The currents 

 in such an estuary are periodic, changing with the flood and 

 ebb, and the conditions of deposition vary accordingly. The 



