462 Geology of the South- East of Dorsetshire. 



44 



portions that remain. According to this supposition, it is 

 evident that the chalk and inferior beds must have origin- 

 ally formed a dome over the whole of Purbeck ; which, tak- 

 ing the height of Ballard Down to be, as he assigns, 

 58 4 ft., could not have been less than 4672 ft. thick. Such 

 an enormous denudation as the present surface must have 



required, on this supposition, is totally at variance with 

 legitimate assumption ; and, therefore, we are called on to 

 reject the original curvature of the chalk altogether. The 

 former idea of curved chalk strata over the slope of horizontal 

 strata, as assumed in Jig. 44., seems to me equally untenable; 

 because, though undoubtedly the chalk was deposited by de- 

 grees, and may have been subjected to the action of currents 

 of water, passing over each successive deposition, there is no 

 example on record, that I am aware of, where the deposition 

 of chalk was interrupted by any long interval, or where it 

 attained a sufficient elevation above the sea to be subjected to 

 denuding causes capable of so rounding and excavating the 

 horizontal beds, and then became depressed, after that long 

 interval, beneath the same depositing sea, whose action was 

 again continued. That different modifications may have oc- 

 curred in the different portions of the chalk formation, is, no 

 doubt, consistent with facts; for how otherwise are we to 



