94 PAl'EKS, ETC. 



runnlng parallel with the Mendip hüls, and is overlaid with 

 beds of blue and white lias rock. This indurated marl is 

 seen on the sides of the deep Channels of the rivers Axe 

 and Parret, near their exit into the sea. 



3. The filling up of these marshes next claims our atten- 

 tion, and here we find natural causes operating in a variety 

 of ways to effect it. The waters of the Bristol Channel 

 are remarkable for their muddy appearance. This arisea 

 from the beds of clay over which they flow, situated at 

 the entrance and along the bed of the river Severn, and 

 other streams which flow into it from the clay soils of 

 Somersetshire. The w\aters thus saturated with clay 

 being kept in constant agitatlon by the tides, which here 

 flow with great rapidity, and rise to the height of from 

 forty to fifty feet perpendicular, necessarily leave a great 

 deposit of alluvial matter, Avhenever its current becomes 

 impeded, or its motion retarded and stopped. This Sedi- 

 ment being exposed to the influeuce of the sun and wind, 

 during the interval between the ebb and flow of the tide, 

 becomes in a certain degree hardened, and receives the 

 deposit of the next tide ; and so by repeated accumulations 

 the whole becomes Consolidated and in tinie fit for the pur- 

 poses of Vegetation. It may further be remarked, that the 

 difference in the height of the spring and neap tides leaves 

 a considerable space of its shore dry for several days. 

 During this interval, aquatic plants and grasses grow up, 

 and on the return of the next spring tides become a recep- 

 tacle for the subsequent deposits, which increase so rapidly 

 as soon to form banks above the level of the oi'dinary tides ; 

 which in the course of time become the barrier against itself, 

 ßo that the highest tides only pass over it. The con- 

 Bequence of this barrier was, it converted the low lands of 

 the interior into a lake, or morass, covered during the 



