THE SEYEEN AND ITS TRIBUTAEIES. 613 



During this period a scour of the bottom is of course going on ; but, 

 as this velocity is not much higher than in the subsequent period of 

 low-water flow, the rate of scour will not be much greater ; and the 

 actual scour will be insufficient to compensate for the amount of deposit 

 from the tidal waters which has taken place, not only during the 

 period of high water, but also during the first two thirds of the ebb. 

 It must follow therefore that the scouring effect of the tide is little or 

 nothing, and the observed incapacity of tidal flows to sweep away 

 the silt they have deposited is amply and satisfactorily explained." 



Though some of the silt in the tidal water may, as thus explained, 

 stay behind in the estuaries themselves, yet the greater portion is 

 carried seaward ; for, in addition to the oscillating movement of the 

 tidal water, there is of course a discharge into the sea of as much 

 water as the rivers bring down into the estuary, and this is probably 

 accompanied by a transference to the sea of a corresponding quantity 

 of suspended mud, so that the final resting-place of the sediment of 

 the Severn is situated some distance out to sea. But between this 

 quiet spot and the margin where the Severn meets the tide the sedi- 

 ment is carried up and down, far and frequently, and it is not till 

 many journeys are accomplished that it comes permanently to 

 rest. 



Thus in the waters of the Severn estuary there is a storage of 

 suspended sediment, the accumulation of as many days, or weeks, 

 or months as are occupied in its wanderings to and fro. The accu- 

 mulation is always being diminished by withdrawals seaward, and 

 as constantly renewed by fresh accessions provided by the denuda- 

 tion of the land. 



This is the whole explanation of the remarkable turbidity of the 

 estuarine Severn. 



Microscopical Examination of the Tidal Mud. — With a view to 

 throwing some additional light on the sources of the Severn silt, I 

 have examined under the microscope specimens from a large number 

 of localities on both sides of the Severn, including its tributaries ; 

 thus Weston- super-mare, Penarth, Portishead, Avonmouth, and 

 Gloucester are sufficiently far apart for the sampling of the Severn 

 itself; Bridgewater served for the Parrot, Rhymney Bridge (near 

 Cardiff) for the Rlrymney, Newport, Mon., for the Usk, Chepstow for 

 the TVye. and Rownham Perry, near Bristol, for the Avon. 



The character of the mud from all these places is so similar that 

 a description of one would serve for all the rest. 



The ingredients may be classed as mineral and organic ; the 

 former consist of : — a variable quantity of fine argillaceous granules, 

 small angular fragments of colourless transparent quartz containing 

 numerous minute included cavities, a few similar fragments of flint, 

 siliceous fragments of a glauconitic green colour, minute crystals 

 of quartz of the ordinary form, minute prisms of tourmaline, highly 

 dichroic and similar in form to macroscopic prisms of schorl, and 

 minute rhombohedra of calcite. 



The organic constituents are siliceous and calcareous, the latter 

 include : — coccoliths and rarely coccospheres, both of the ordinary 



