CHAP. XIII 
THE LAKE DISTRICT 
229 
thickest accumulation of volcanic material, belonging to a single geological 
period, anywhere known to exist in Britain. 
The geological age of this remarkable volcanic episode is fortunately 
fixed by definite palaeontological horizons both below and above. The base 
of the volcanic group rests upon and is iuterstratified with the upper part 
of the Skiddaw Slate,^ which from the evidence of its fossils is paralleled with 
the Arenig rocks of Wales. The highest members of the group are inter- 
stratified with the Conistoii I.imestone, 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 tliat elapsed betw'een 
the later part of the Arenig period and the close of the Bala period. It 
begins probably not so far back as that of the Arenig group of Merioneth- 
shire, and its termination was perhaps coincident with the dying out of the 
Snowdonian volcanoes. But it contains no record of a great break or 
interval of quiescence like that which separated the Arenig from the Bala 
eruptions in Wales. 
The materials that form tliis enormous volcanic pile consist entirely of 
lavas and ashes. Xo intercalations of ordinary sedimentary material have 
been met with in it, save at the bottom and at the top. The lower lavas, 
well seen among the hills to the south of Keswick, were shown by Mr. 
Ward to be intermediate betw'een felsites and dolerites in regard to their 
silica percentage, and he proposed for them the name of felsi-dolerites. Ihey 
are comprised in the group of the andesites or “ porphyrites. h rom the 
analyses published Ijy Mr. W^ard, tlie amount of silica appears to range up 
to about 60 per cent." They are usually close-grained, dull dark-grey to 
black rocks, breaking, where fresh, with a splintery or conchoidal fracture, 
showing a few minute striated felspars, apt to weather with a pale-brown or 
yellowish-gi'ey crust, and sometimes strongly vesicular or amygdaloidal. 
'They present many external resemblances to some of the “ porphyrites or 
altered andesites of the Lower Old Bed Sandstone of Scotland. A micro- 
scopic examination of specimens collected by Ur. Hatch and myself from the 
hills to the south of Keswick showed the rocks to be true andesites, composed 
of a multitude of slender laths (sometimes large porphyritic crystals) of 
* Mr. Dakyus lias expressed Iris belief that the volcanic group lies unconformahly on the 
Skiddaw Slate (Geol. Mag. 1869, pp. 56, 116), and Professor Nicholson has formed the same 
opinion (o;;. cil. pp. 105, 167 ; Free. deal. Asme. vol. iii. p. 106). ^ Mr. Goodchild, however, has 
shown that in the Cross Fell inlier the oldest tuffs are interstratiiied with the Skiddaw Slates 
{Proc. deal. Assoc, vol. xi. (1889), p. 261). Mr. Ward in mapping the district inserted a complex 
series of laults along the junction-line between the volcanic series and the Skiddaw Slates. When 
I went over the ground with him some yeans before his death I discussed this boundary-line with 
him and could not adojit his view that it was so dislocated. More recent re-examination has con- 
firmed me in my dissent. A large number of the faults inserted on the Geological Survey map to 
separate the .Skiddaw Slates from the Uorrowdale volcanic series cannot be proved, and probably 
do not exist. Others may be of the nature of “thrust-planes.” But see Mr. AVard’s explanation 
of his views, op. cil. p. 48. 
- Quart. Jmira. Gcol. Soc. vol. xxxi. (1875) p. 408, vol. x.xxii. (1876) p. 24. Geology of 
Northern Part of Lake District {Mem. deal. Survey), p. 22. In a subseiiueut papew the more basic 
lavas of Eycott Hill are compared with dolerites (Monthly Microm>pical .Jmirn. 1877, p. 246). 
