332 
THE SILURIAN VOLCANOES 
BOOK IV 
part of the Conistou Limestone, and spreads out underneath it along the 
southern margin of the volcanic district from the Shap granite south-westward 
lor some miles ^ (Lig. 62). Between the valleys of the Sprint and Kent 
these felsites (which farther east are said to be 700 feet tliick) may be seen 
interposed between the limestone and the fossiliferous calcareous shales 
below It, wliile trom underneath the latter other sheets rise up into the raiic^e 
of hills behind. 
These acid lavas are generally grey, cream-coloured, or pink, with a 
white weathered crust. Their texture when fresh is flinty or horny, or at 
least extremely fine-grained and compact. They are seldom markedly 
porphyritic. They frequently display good flow-structure, and sometimes 
spht up readily along the planes of flow. Occasionally the flow-lines on the 
outer crust have broken up in the movement of the rock, giving rise to 
irregular fragments which have been carried forward. Short, extremely 
irregular, branching veins of a flue chert}- felsitic substance, which occa- 
sionally shows a well-marked flow-structure parallel to the walls, traverse 
certain parts of a dark-gi'ey felsite, near Brockstones, between the valleys of 
J IG. 62. Section of felsites on the Coulston Limestone group, we.st of Stockdale. 
n, Felsites more or lesi; clraved ; h, Calcareous shales with fossils, much cleared; c, Cleaved felsite • 
rf, Conistoii Limestone ; c, Stockdale Whales (with graptolites). 
the Kent and Sprint.^ Occasionally a distinct nodular structure may be 
observed in these acid lavas, sometimes minute, like an oolite, in other parts 
as on Great Yarlside, presenting large rounded balls. This nodular structure 
IS not confined to the lava-flows, but has been detected by Messrs. Harker 
and Marr in what appears to be an intrusive rock near Shap Wells. The 
microscopic characters of some of the Lake District rhyolites were described 
by Mr. Eutley, who found them to exhibit beautiful perlitic and spherulitic 
structures.® That such rocks as these were poured out in a vitreous condi- 
tion, like obsidian or pitchstone, cannot be doubted. Chemical analysis 
shows that the Lake District rhyolites agree exactly with those of North 
Wales in their composition. They contain about 7 6 per cent of silica.'^ 
1 he rhyolitic lavas have been seriously affected by the general cleavane 
of the region. In some places they have been so intensely cleaved as to 
ecome a kind of fissile slate, and there seems good reason to believe that in 
^ Unfortunately these 
Survey maps. 
acid lavas are uot distinguished from the others in the Geological 
- Compare the structure described by Mr. Harker fr 
S<K. xlvii. (1891), p. 518. 
om the Cross Fell inlier, Quart. Journ. Geol. 
:i 
4 
‘‘Geology of Kendal,” etc., Mem. Ocol. Survey, Sheet 98 N.E. 2nd edit. 
Messrs. Harker and Marr, op, dt, p. 302. 
p. 9. 
