SOME ANCIENT AND MODERN VOLCANIC EOCICS. 413 



that of the slaggy felstones of Wales. Nor need we suppose that the 

 ash-rocks were in any sense rendered fluid, in order to produce this 

 streaky structure ; for it is evident that in the case of the old lava 

 of Snowdon (fig. 20), the flowing matter could only have been 

 slightly viscous, or otherwise such fragments as the large one 

 represented in the figure would have been moved round so that their 

 longer axes would lie in the direction of the bands, instead of these 

 last curving round them as they do. 



In some places the original lines of bedding are apparent, as well 

 as the streaky structure ; and in such cases it is usual to find that 

 the two agree in direction, as if the alteration had taken place 

 more readily along such planes than along any others. I am there- 

 fore led to conclude that in all probability there are no contem- 

 poraneous felstones, answering to those of Wales, in the Cumberland 

 district, but that the felstone-like rocks, which present none of the 

 characters of the Cumberland traps already described as such, are 

 the result of metamorphism taking place when buried deeply beneath 

 overlying rocks, since removed by denudation, just as the chiastolite 

 slate and spotted schist, surrounding the Skiddaw granite, are the 

 results of a similar metamorphism, probably effected at the same 

 period*. 



d. Period of metamorphism. — It seems probable that the period 

 during which this metamorphism chiefly took place was that of the 

 close of the Upper Silurian or earlier part of the Old Ked, and for 

 these reasons: — 1. The Upper Old Red or basement-beds of the 

 Carboniferous series (conglomerate of Mell Pell and Pooley Bridge) 

 rest unconformably on both Skiddaw slates and the Volcanic series. 

 2. Therefore the Volcanic series had been denuded from the present 

 northern area of Skiddaw slate previous to the deposition of this con- 

 glomerate. 3. The Upper Silurians in all probability once extended 

 over the Volcanic series before these last were denuded. 4. Mr. 

 Aveline estimates the Upper Silurian in the Kendal district as at 

 least 14,000 feet thick ; so that at the close of this period the volcanic 

 rocks were probably buried at a very great depth, the granite being 

 partly formed or intruded amongst them. 5. During the whole 

 of the Old Red period this district was being slowly raised, and 

 denuded by the sea and atmospheric powers ; and this is very likely 

 the reason why the Old Eed is here unrepresented by deposits except 

 quite at its close or at the commencement of the Carboniferous. It 

 would be an interesting thing to investigate this subject further, by 

 a thorough examination of the evidence afforded by the comparative 

 size of the liquid- cavities and their vacuities in the quartz of the 

 granites, in the manner Mr. Sorby has so ably done in other cases ; 

 and when we consider his results, viz. " that the fluid-cavities indi- 

 cate that all the elvans and granites hitherto examined were con- 

 solidated under pressures varying from about 18,000 to 78,000 feet 



* There is, however, a true felstone bed, showing lines of viscous flow, in 

 connexion with the Coniston Limestone, west of Shap; and the difference 

 between this and most of the altered rocks described above is very marked, both 

 lithologically and microscopically. 



