230 
THE SILURIAN VOLCANOES 
BOOK IV 
lelspar with a brownish glassy groundmass, and with some chloritic material 
probably representing angite, but with no trace of quartzd 
Another type of andesite lias been found by Mr. Hutchings to occur 
abundantly at Harter Fen, Mardale, between the Nan Bield Pass and High 
Street, and in the cliffs on the right side of the Kentniere Valley. It con- 
sists of roclvs mostly of a grey-green or grey-blue colour with resinous lustre 
and extremely splintery fracture. They are augite-andesites of a much more 
vitreous nature than the dominant type of lavas of the Lake Histrict. Their 
groundmass under the microscope is seen to have originally varied from a 
wholly glassy liase to an intimate mixture of glass and exceedingly minute 
felspar-microlites. This groundmass is permeated with chlorite in minute 
hakelets, and encloses numerous porphyritic sharply-detined felspar-crystals, 
together with chlorite-pseudomorphs after angite.- flradations fr om these 
rocks to the ordinary more coarse-grained andesites may be observed. 
Some of the andesites appear to have a trachytic facies, where the 
felspars of the groundmass consist largely of untwinned laths and appear to 
be mainly orthoclase.® 
Among the lavas of the Lake District there occur many which are 
decidedly more basic tlian the andesites, and which should rather be classed 
among the dolerites and basalts, though they do not appear to contain olivine. 
These rocks occur at Eycott Hill, above Easedale Tarn, Scarf Gap Pass, Dale 
Head, High Scawdell, Seatoller Fell and other places. Analyses of those 
from Eycott Hill were published by Mr. Ward, and tlieir silica percentage 
was shown to range from 51 to 53‘3.'* The microscopic characters of the 
group have been more recently determined by Mr. Hutcliings ° and Messrs. 
Harker and iMarr.^ 
The andesitic and more basic lavas are particularly developed in the 
lower and central part of the volcanic group. They rise into ranges of 
craggy hills above the Skiddaw Slates, and form, with their accompanying 
tuffs, the most rugged and lofty ground in the Lake District. They extend 
even to the southern margin of the volcanic area at one locality to the south- 
west of Coniston, where they may be seen with their characteristic vesicular 
structure forming a succession of distinct hows or beds, striking at the 
Coniston Limestone which lies upon them with a decided, though probably 
veiy local, unconformability.' Gne of the Hows from this locality was found 
by Dr. Hatch, under the microscope, to belong to tlie more basic series. It 
’ Tliesc rocks were mapped as tuffs by Mr. Ward. TJieir microscopic eliaracters liave been 
described by Messrs. Harker and Marr, Quart. Journ. Gcol. Soc. xlvii. (1891), p, 292 ; by Mr. 
Harker, op. fit. p. 517 ; and by Mr. W. M. Hutchings, Gcol. JIari. 1891, p. 537 ; 1892, pp. 227, 540. 
Mr. Hutcliings, Gcol. Mag. 1891, p. 539. Tins observer describes a quartz-andesite or dacite 
from near Duiuuail Raise. » tjp, fit. p. 543 
■» .Monthly Mkromjpkal Journal, 1877, p. 246. ■' Gcol. Mag. 1891, p. .538. 
^ Quart. Jmrn. Gcol. tioc. vol. xlix. (1893), p. 389. .Mr. Harker, op. cit. vol. xlvii. (1891). 
' lids uuconforinability lias been described and discussed by various observers. The general 
impression has been, 1 think, that tlie break is only of local importance. Mr. AveUne, howevei-, 
believed it to be much more seiious, and he regarded the volcanic rocks which were ejected durinv 
the deposition of the Coni.stou Limestone series a.s much later in date than those of the Borrowdale 
group. See Man. tlrol. Surccy, Exiihinatiou to Sheet 98 N.E. 2nd edit. p. 8 (1888). 
