44 
It is in connection with these masses of blue diabase, and the dyke 
of diorite, that the rich deposits of plumbago have been found. 
There is a small exposure of diorite on Swirral Edge, Helvellyn. 
The rock is very hard and compact, and consists of small crystals 
of plagioclase and orthoclase felspar, augite, and magnetite, also a 
few scattered cubes of iron pyrites. 
The Armboth Dyke is a quartz felsite, being precisely the same 
in chemical composition as the St. John’s Quartz Felsite. It is a 
very beautiful rock, consisting of a dull red felsitic base, studded 
with numerous crystals of pink felspar and transparent quartz, also 
a little serpentine, and occasional grains of green mica. The dyke 
is from twenty to thirty feet in width, and extends in a NN.W. and 
SS.E. direction across Armboth Fell, where it may be traced for a 
distance of about one and a half miles. It appears again on the 
opposite side of the valley, near the seventh milestone on the road 
from Keswick to Ambleside, and extends in a S.E. direction beyond 
the crest of Helvellyn. 
There are several branching dykes stretching across the upper 
part of Langstrath valley, in Borrowdale. The rock of which they 
are composed consists of crystals of felspar embedded in a felsitic 
base. There is a good deal of chloritic matter disseminated 
through the base, which also contains some augite, magnetite, and 
pyrites. Occasionally the mass is traversed by veins of red felsitic 
material. There are also dykes on Scafell, Kirkfell, Watendlath 
Fell, and others of minor importance. 
The sheets of lava that are interstratified with the ashes and 
breccias of the volcanic series, are mostly very hard and compact, 
of a grey or bluish green colour, and they often contain crystals of 
augite, quartz, calcite, and other minerals. The best section of 
lavas and ashes to be found in the whole series occurs on the 
eastern shore of Derwentwater, extending from the margin of the 
lake to the summit of Bleaberry Fell. In this section there are 
twelve distinct sheets of lava, which vary in thickness from fifteen 
to one hundred and fifty feet, the upper and under surface in each 
case being more or less vesicular and cinder-like, while the 
remainder is massive and compact. One of the number, which 
