52 Rev. J. Clifton Ward — Geology of the Lake District. 



B. — Volcanic Period. 



The rocks deposited in the present area of the Lake District during 

 the period coming between that of the Skiddaw Slates and that of 

 the Upper Silurian are almost exclusively of Volcanic origin. They 

 may represent a total thickness of about 12,000 feet. At the base of 

 the volcanic series only are there intermixtures with rocks of an 

 ordinary sedimentary character ; here, where the junction beds are 

 exposed, occur alternations of Skiddaw Slate and submarine volcanic 

 deposits. The rest of the series consists of beds of volcanic ash and 

 breccia with lava flows. The finer ash deposits are frequently 

 well stratified and false-bedded. The breccia is of all degrees of 

 coarseness, from a rock made up of fragments having the size of a 

 sixpence or shilling to one containing blocks several yards in 

 diameter. Conglomeratic ash occurs in one or two beds near the base 

 of the series. Much of the variation in appearance among the beds 

 of the ashy series is due to subsequent alteration, metamorphic 

 action producing diverse changes, dependent, oftentimes, upon slight 

 original differences in texture and composition — examples of selective 

 metamorphism. The lava-flows are either good dolerites and basalts, 

 or belong to a class more or less mediate between these and the more 

 acidic group of lavas. As is generally the case among volcanic 

 deposits, the various beds are more or less irregular in their range, 

 showing instances of rapid thickening and thinning. 



Physical Conditions indicated. — The presence of ordinary sedi- 

 mentary beds interstratified among the volcanic deposits near their 

 base ; the occasional occurrence in this lower part also of con- 

 glomeratic ash ; and the absence of both these peculiarities in the 

 great bulk of the volcanic series, together with that of fossils in 

 the bedded ashes, — all point to volcanic action commencing at the 

 close of the so-called Skiddaw Slate period beneath the waters 

 of the Skiddaw Slate sea, and the gradual passage from sub- 

 marine volcanic conditions to those of terrestrial and wholly sub- 

 aerial volcanos. At first sight it might seem that the regularly- 

 bedded ashes running at intervals throughout the series, pointed to 

 subaqueous deposition, but no one can ramble much around modern 

 terrestrial volcanos without being struck by the frequent cases of 

 fine stratification shown by the ash scattered around, whether de- 

 posited in the wet or dry state, and by the not infrequent cases of 

 false-bedding. It may sometimes have happened also that extensive 

 deposits of ashy material were laid down in large crater-lakes. 



The centres of eruption are difficult to fix upon, as might be 

 expected amongst volcanic remains of such antiquity. The boss of 

 Castle Head, Keswick, almost certainly represents one such centre, 

 and the best developments of lava-flows are all found occurring 

 within an easy distance. It may be further remarked that since the 

 lower part of the series contains the greatest thickness of lava-flows, 

 it would seem that the chief emissions of lava were followed by 

 long and continued ejections of ashy material. What the height of 

 the old Cumbrian volcano or volcanos may have been, it is difficult 

 to estimate ; but volcanic deposits were accumulated to a thickness, in 



