CHAP. VIII. RIVERS SHIFTING THEIR CHANNELS. 129 



Changes in a river-plain, such as those above alluded to, 

 give rise frequently to ponds, swamps, and marshes, marking 

 the course of old beds or branches of the river not yet filled 

 up, and in these depressions shells proper both to running 

 and stagnant water may be preserved, and quadrupeds may 

 be mired. The latest and uppermost deposit of the series 

 will be loam or brick-earth, with land and amphibious shells 

 (Helix and Succinea^, while below will follow strata contain- 

 ing fresh-water shells, implying continuous submergence; 

 and lowest of all in most sections will be the coarse g-ravel 

 accumulated by a current of considerable strength and 

 velocity. 



When the St. Katharine docks were excavated at London, 

 and similar works executed on the banks of the Mersey, old 

 ships were dug out, as I have elsewhere noticed,* showing 

 how the Thames and Mersey have in modern times been 

 shifting their channels. Recently, an old silted-up bed of 

 the Thames has been discovered by boring at Shoeburjmess 

 at the mouth of the river opposite Sheerness, as I learn from 

 Mr. Milne. The old deserted branch is separated from the 

 new or present channel of the Thames by a tertiary outlier 

 composed of London clay. The depth of the old branch, or 

 the thickness of fluviatile strata with which it has been filled 

 up, is seventy-five feet. The actual channel in the neigh- 

 borhood is now sixty feet deep, but there is probably ten or 

 fifteen feet of stratified sand and gravel at the bottom ; so that, 

 should the river deviate again from its course, its present bed 

 might be the receptacle of a fluvio-marine formation seventy- 

 five feet thick, equal to the former one of Shoeburyness, and 

 more considerable than that of Abbeville. It would consist both 

 of fresh-water and marine strata, as the salt water is carried by 

 the tide far up above Sheerness; but in order that such de- 



••■■■ Principles of Geology. 



