126 GEOLOGY. 



specimens of incrustation are reported to have been obtained 

 from its source, I have been unable to detect anything of the 

 kind, in either the water-courses or the soil over which it flows. 



The problem then which has to be solved is the presence and 

 consequent introduction of marine shells. Mr. Mansel endeavours 

 to do this through an argument which maintains the probability 

 of the occurrence of frequent changes of level, such depressions 

 and upheavals of the district, as to admit and throw back, 

 alternately, the estuary waters of Poole harbour, through the 

 gorge at Corfe. Now I contend that there are insuperable dif- 

 ficulties in the way of the establishment of such a theory. In 

 the first place, if this varied change of level be admitted, we 

 expect to find, agreeably not only with the laws of geological 

 science, but also with those of natural evidence, a series of strata, 

 fresh water and marine, alternating with each other. Besting 

 upon the Hasting's sands there would be a certain thickness of 

 tufa containing land and freshwater shells only; then would 

 succeed evidence of the influx of the sea, in a stratum of silt 

 containing for the most part marine exuviae : then again, when 

 the sea retired and the fresh water obtained repossession, there 

 would be another bed of tufa, and so on, if indeed it can be 

 supposed that these sudden upheavals, and equally sudden de- 

 pressions would have been so equal and regular in their action, 

 as to permit so complete an integrity of the general features of 

 the surface, that after each, the same spring should reform a 

 lake over precisely the same area as it had previously done. 

 But there is not the slightest indication of stratification: on the 

 contrary, the bed presents a uniform mass of concretion, and 

 the marine shells occur intimately mingled with the land. Mr. 

 Mansel suggests that the land shells might have been picked up 

 by the sea as it passed over what had previously been dry land, 

 and hence conveyed. Still, if we grant this to be possible, we 

 should expect to find a diflference both in the composition and 

 colouring of the matrix in which they became embedded, which 

 is not the case. And they would be discovered, not only in one 

 isolated spot, but frequently over the whole district which the 

 sea must have inundated, including all the low lands of the Isle 

 of Purbeck, and the tertiary valley as far as Dorchester ; Blash- 

 enwell being upwards of a hundred and ten feet above its level. 

 The lenticular deposit lately discovered in the Church-yai'd, at 



