234 
THE SILURIAN VOLCANOES 
KOOK IV 
Fig. t)3. — Fine tuff ivitli coarser bands near Quayfoot quarries, 
Borrowdale. 
The liighly-incliiied line lines show the cle.ivaf;e. The more gently ilinpiii" 
hjimls and lines mark the bedding. ° 
group some of the tuffs abound in blocks and chips of Skiddaw Slate. 
Some good examples of this kind may be seen in Borrowdale, below Falcon 
Crag and at the Quayfoot tptarries. Where the tuff is largely made up of 
fragments of dark blue 
slate, it much resembles 
the slate-tuffs of Cader 
I dris. Some of the pieces 
of slate are six or eight 
inches long and are now 
placed parallel to the 
cleavage of the rock. 
Among the slate debris, 
however, felspar crystals 
and felsitic fragments 
may lie observed. Bands 
of coarser and liner green tuff show ^nry clearly the bedding in spite of 
the marked cleavage (Fig. 03). 
But throughout the whole volcanic group the material of the tuff is 
chielly of thoroughly volcanic origin, and its distribution appears to agree on 
the whole with that of the bedded lavas. In the older portions of the group 
it is probably mainly derived from andesitic rocks, though with an occasional 
intermingling of felsitic or rhyolitic detritus, while in the higher pai’ts many 
ol the trrffs are markedly rhyolitic. Among the lapilli minute crystals of 
felspar, broken or entire, may be detected with the microscope. Some 
of the ejected ash must have been an exceedingly fine dust. Compacted 
layers of such material form bands of green slates, wlrich may occasionally 
be seen to consist of alternations of coarser and liner detritus, now and then 
false-bedded. Such tuffs bring vividly before the mind the intermittent 
explosions, varying a little in intensity, by which so much of the fabric of 
the Lake mountains was built up. 
Breccias of varying coarseness are likewise abundant, composed of frag- 
ments of andesite and older tuffs in the central and lower parts of the volcanic 
group, and mainly of felsitic or rhyolitic detritus in the upper parts. Some 
of these rocks, wherein the blocks measure several yards across, are probably 
not far from the eruptive vents, as at Sourmilk Gill and below Honister 
Bass. Generally the stones are angular, but occasionally more or less 
rounded. Stratilication can generally be detected among these fragmental 
rocks, but it is apt to be concealed or effaced by the cleavage, wliile it is 
furtlier obscured by that widespread induration on which Mr. Ward has 
laid so much stress. Tlie extreme state of comminution of the volcanic 
dust that went to form tlie tuffs lias probably caused them to be more 
liable to metaniorphism than the lavas.’ 
Little has yet been done in identifying any of the vents from which 
the vast mass of volcanic material in the Lake District was ejected. Mr. 
' 1 he inicroseopic and chemical characters of the Ash-Slatcs of the Lake District have been 
investigated by Mr. Hntchiug.s, (Rol. May. 1892, pp. 155, 218. 
