32 AQUEOUS AGENCIES. 



immense waves, rushing along like an advancing cataract. This is 

 called an eagre or lore. The finest examples are perhaps in the Amazon 

 and Tsien-tang Eivers. In the eagre of the Amazon " the tide passes 

 up in the form of three great waves, thirteen to twenty-three feet 

 high." * In the Tsien-tang, a single wave plunges along at the rate of 

 twenty-five miles an hour,f with perpendicular front, like an advancing 

 cataract, four or five miles wide and thirty feet high. In the river 

 Severn also we have a remarkable example of an eagre. According to 

 the laws already developed (pp. 19 and 20), the erosive and transport- 

 ing power of such currents must he immense. 



Deposits in Estuaries. — The larger portion of the materials thus 

 eroded is carried out to sea by the retreating tide, and will be again 

 spoken of under " Sea-deposits." A portion of these materials, how- 

 ever, is always deposited in the estuary in sheltered coves and bays 

 (Fig. 24, a and b), and often, when the outflowing tide is obstructed by 

 sand-spits and islands at the mouth, over every portion of the estuary. 

 In addition to this, especially in rivers subject to great freshets, there 

 are deposits of silt from the river. Thus many estuaries are occupied 

 alternately, during the wet and dry seasons, by fresh and brackish or salt 

 water, and the deposits in them are therefore alternately fresh-water 

 and salt-water deposits, containing fresh -water and salt or brackish 

 water shells. These alternations are highly characteristic of estuary- 

 deposits in all geological periods ; in fact, of all deposits at the mouths 

 of rivers where river and ocean agencies meet. 



8.— Bars. 



Bars are invariably formed in accordance with the law already 

 enunciated as that controlling all current-deposits, viz., if the velocity 

 of a current bearing sediment be checked, the sediment is deposited. 



There are two positions in which bars are formed : 1. At the mouths 

 of rivers ; and, 2. At the head of the estuaries. In the first position 



Fig. 24.— An Estuary. 



* Branner, Science, 1884, vol. iv, p. 488. 



•j- American Journal of Science and Arts, 1855, vol. xx, p. 305. 



