228 
THE SILURIAN VOLCANOES 
BOOK IV 
appeared. The sheets of the Geological Survey map present a graphic 
view of the general distribution of the rocks, hut so rapid has the progress 
of certain branches of geology been since these sheets were published, that 
the map is even now susceptible of considerable improvement. 
In estimating the area over which the volcanic rocks of the Lake 
District are spread, geologists are apt to consider only the tract which lies 
to the south of Keswick and stretches southward to a line drawn from the 
Duddon Sands to Shap. But it can easily be shown that this area falls 
far short of the extent of that wherein the rocks can still be traced, and 
yet further short of that over which the lavas and ashes originally spread. 
For, in the first place, the volcanic group can be followed round the eastern 
end of the mountain-group which culminates in Skiddaw, and along the 
northern base of these heights to Cockennouth, though only a narrow fringe 
of it emerges from underneath the Carboniferous series. It is thus manifest 
that the volcanic rocks once stretched completely across Skiddaw and its 
neighbours, and that they extend northwards below the Whitehaven Coal- 
field. But, in the next place, far beyond these limits, volcanic rocks, which 
there can be little doubt were originally continuous with those of the Lakes, 
emerge from beneath the base of the Cross Fell escarpment,^ and still further 
to the east a prolongation of the same group rises for a brief space to the 
surface from under the great limestone sheets of Upper Teesdale. Between 
the north-western and south-eastern limits within which the rocks can now 
be seen there intervenes a distance of some 11 miles, while the extreme 
length of the tract from south-west to north-east is about 50 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 proportionate to 
the large tract of country over which they have been spread. From 
various causes, it is difficult to arrive satisfactorily at any precise statement 
on this question. In a volcanic series bedding is apt to be obscure where, 
as in the present case, there are no interstratified bands of ordinary sedi- 
mentary strata to mark it off. It tends, moreover, to vary considerably and 
rapidly within sliort distances, not only from subsequent unequal move- 
ments 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 
to 15,000 feet." Professors Harkness and Kicholson, on the other hand, 
gave the average thickness as not more than 5000 feet.® ]\Iy own impres- 
sion is that the truth is to be found somewhere between these two esthnates, 
and that the maximum thickness probably does not exceed 8000 or 9000 feet. 
In any case there cannot, 1 think, be much doubt that we have here the 
’ For ail account of the Cros-s Fell inlier of Silurian rocks see the paper by Professor Xicholsoii 
and Mr. Marr, with the petrographical appendix by Mr. Harker. Quart. Journ. Geol, Soc. vol. 
xlvii. (1891), pp. 500, 512. 
- Ward, op, dt. p. 46. 
Brit. Assoc. (1870) Sectional Itcports, }>. 74. 
