CHAP. XIII 
THE LAKE DISTRICT 
233 
this altered condition they have often been mistaken for tuffs. Where they 
assume a nodular structure, the nodules have sometimes been flattened and 
elongated in the direction of the prevalent cleavage. 
Tlie abundance and persistence of thoroughly acid lavas along the 
southern edge of the volcanic area where the youngest outflows are found, 
is a fact of much interest and importance in the history of the eruptions of 
this region. It harmonizes witlr the observations made in Wales, where in 
the Arenig, and less distinctly in the Bala group, a marked increase in acidity 
is noticeable in tlie later volcanic products. At the same time, as above 
mentioned, there is evidence also of the discharge of more basic materials 
towards the close of the eruptions, and even t)f the outflow of a lava 
approaching in character to basalt. 
According to the Geological Survey maps, by far the largest part of the 
volcanic district consists of pyroclastic materials. When my lamented friend, 
the late Mr. Ward, was engaged in mapping the northern part of the district, 
which he did with so much enthusiasm, 1 had an opportunity of going over 
some of the ground with him, and of learning from him his ideas as to the 
nature and distribution of the rocks and the general structure of the region. 
I remember the difficulty 1 had in recognizing as tuff much of what he had 
mapped as such, and I felt that had I been myself recpiired, without his 
experience of the ground, to map the rocks, I should probably have greatly 
enlarged the area coloured as lava, with a corresponding reduction of that 
coloured as tuff. A recent visit to the district has revived these doubts. It 
is quite true, as Mr. Ward maintains, that where the finer-grained tuffs have 
undergone some degree of induration or metamorphism, they can hardly, by 
any test in the field, be distinguished from compact lavas. He wa.s himself 
quite aware of the objections that might be made to his mapping,^ but the 
conclusions he reached had been deduced only after years of unremitting 
study in the field and with the microscope, and in the light of experience 
gained in other volcanic regions. Nevertheless I think that he has some- 
what exaggerated the amount of fragmental material in the northern part of 
the Lake District, and that the mapping, so consistently and ably carried oirt 
by him, and followed by those members of the Survey who mapped the rest 
of the ground, led to similar over-representation there. Some portions of the 
so-called tuffs of the Keswick region are undoubtedly andesites; other parts 
in the southern tracts include intercalated bands of felsite as well as andesite. 
But even with this limitation, the pyroclastic material in the Lake 
District is undouljtedly very great in amount. It varies in textme from 
coarse breccia or agglomerate, with blocks measuring several yards across, 
to the most impalpable compacted volcanic dust. In the lower parts of the 
^ Ho says : “I sliall bo very iiiueli surprised if my mapping of many parts of tlie district be 
not severely criticized and found fault with by those who examine only one small area and do not 
take into consideration all the facts gathered together, during the course of several years, from 
every mountain flank and summit ” (up. nit. p. 25). Mr. Hutchings has expressed his agreement 
with the opinions stated in the text. He likewise coincides in the belief that there are many of 
these Lake District volcanic rocks, regarding which it is impossible to decide whether they are 
lavas or ashes (Geo/. Mag. 1S91, p. 544). 
