138 PROCEEDINGS OF THE GEOLOaiCAL SOCIETY. 



from under the great limestone sheets of Upper Teesdale. Eetween 

 the north-western and south-eastern limits within which the rocks 

 can now be seen there intervenes a distance of some eleven miles, 

 while the extreme length of the tract from south-west to north-east 

 is about fifty miles. Even if we take these figures as marking the 

 approximate boundaries of the region covered by the volcanic 

 ejections, it cannot be less than 550 square miles. But this is 

 probably much less than the original area. 



The thickness of the accumulated volcanic materials is propor- 

 tionate to the large tract of country over which they have been spread. 

 From various causes, it is difficult to arrive satisfactorily at any pre- 

 cise statement on this question. In a volcanic series bedding is 

 apt to be obscure where, as in the present case, there are no inter- 

 stratified bands of ordinary sedimentary strata to mark it ofi^. It 

 tends moreover to vary considerably and rapidly within short dis- 

 tances, not only from subsequent unequal movements of subsidence 

 or elevation, but from the very conditions of original accumulation. 

 Mr. Ward considered that the maximum thickness of the volcanic 

 group of the Lake District might be taken to range from 12,000 ta 

 15,000 feet*. Professors Harkness and Mcholson, on the other 

 hand, gave the average thickness as not more than 5000 feet f. My 

 own impression is that the truth is to be found somewhere between 

 these two estimates, and that the maximum thickness probably does 

 not exceed 8000 or 9000 feet. In any case there cannot, I think,, 

 be any doubt that we have here the thickest accumulation of vol- 

 canic material anywhere known to exist in Britain. 



The geological age of this remarkable volcanic episode is fortu- 

 nately fixed by definite palaaontological horizons both below and 

 above. The base of the volcanic group rests upon and is inter- 

 stratified with the Skiddaw Slate j:, which from the evidence of its 

 fossils is paralleled with the Arenig rocks of "Wales. The highest 

 members of the group are interstratified with the Coniston Lime- 

 stone, which from its abundant fauna can without hesitation be 

 placed on the same platform as the Bala Limestone of Wales, and 

 is immediately followed by the Upper- Silurian series. Thus the 

 volcanic history comprises the geological interval that elapsed be- 

 tween the later part of the Arenig period and the close of the Bala 



* Ward, 020. cit. p. 46. t Brit. Assoc. 1870, Sectional Eeports, p. 74. 



:|: Mr. Dakyns, however, beHeves that the volcanic group lies unconformably 

 on the Skiddaw Slate (Geol. Mag. 1869, pp. 56, 116), and Prof. JVicholson has 

 expressed the same opinion {pp. cit. pp. 105, 167 ; Proc. Geol. Assoc, vol. iii.- 

 p. 106). 



