17G GEOLOGY. 



using the term "transitory inroads," I meant to imply high 

 titles, — unusually high. The sea might, under these cir- 

 cumstances have forced its way into the lake by the ancient 

 river course, accompanied by such light materials as limpets, 

 pieces of flint, &c. This does not involve the necessity of a 

 stratum of silt, as much of the lighter portions of the foreign 

 matter would return by the reflux tide before it had time to settle 

 to the bottom. There is an evidence of change in the tufaceous 

 character, where the marine shells are found ; the tufa is less 

 pure and is of a pink hue. Mr. Austen's objection to my 

 hypothesis of the river at unusually high tides becoming a tidal 

 stream, namely, the absence of a stratum of silt, would equally 

 apply to the cause he assigns for the introduction of land shells; 

 for, he says, "after heavy rains, much surface water would 

 flow into the stream, bringing land shells from the hills and con- 

 veying them down its course." If no indication of foreign matter 

 is met with at Blashenwell where the tufa is of the purest 

 character, what has become of the soil, mud, and other bodies 

 which, coming under the the influence of the surface water, 

 flowed into the stream, "bringing land shells" from the neigh- 

 bouring hills ? 



I never spoke of a depression, but merely of an upheaval, of 

 which there are various evidences around the coast, especially 

 in the neighbouring county of Devon. The raised beaches at 

 Torquay and Bideford, are too well known to require description. 

 The Red Sand formation is favorable for the preservation of 

 animals of bye-gone days: the blown sand, in combination with 

 oxide of iron or lime acting as a cement, bind the shells into 

 a kind of conglomeration, from which it requires a sharp blow of 

 the hammer to dissever them. 



It is universally admitted, that in a portion of our island there 

 have been great movements of elevation subsequent to the cre- 

 ation of existing mollusca. If we ascend the estuary of the 

 Severn as high as Shropshire, we And sea shells of existing 

 species from three to six hundred feet above the level of the sea. 

 The character of the rocks which form the Dorsetshire coast is 

 not favorable to the preservation of the old coast line, and it is 

 difiicult to trace it. On one side, the limestone rocks are too 



