CHAP, xm 
THE LAKE DISTRICT 
235 
Ward believed that the diabase boss forming the Castle Head of Keswick 
marks the site of “ one of the main volciinic centres of this particular dis- 
trict”^ whence the great lava sheets to the southward bowed out. There 
are obviously two groups of bosses on the northern side ol the district, some 
of which may possibly mark the position of vents. A lew of them are 
occiipied by more basic, others by more acid rocks. It is not necessary 
to suppose that the andesitic lavas ascended only from tlie former and the 
felsites from the latter. While the telsites on the whole are younger than 
the more basic lavas, they may have been erupted from vents which had 
previously emitted andesites, so that the present plug may represent only 
the later and more acid protrusions. 
Besides the boss of Castle Head there are numerous smaller basic 
intrusions farther down the Derwent Valley on either side of Bassenthwaite 
Lake. Among these are the highly basic rocks forming the picrite on the 
east side of the Dash Beck and the dykes on Bassenthwaite Common. All 
these bosses, sills, and dykes rise through the Skiddaw Slates, but there is 
no positive proof that they belong to the Lower Silurian volcanic series ; 
they may possibly be much later. 
The most important and most interesting of all the intrusive masses 
of basic material is that which constitutes a large part of the eminence 
that culminates in Carrock Fell. The remarkable variations in the 
composition of this mass have been already referred to. Mr. Harker has 
shown that while the centre ot the mass is a quartz-gabbro, it becomes 
progressive!}' more basic towards the margin. Through the gabbro a mass 
of granophyre has subsequently made its way, and along the line of junction 
has incorporated into its own substance so much ot the basic rock as to 
undergo a marked modification in its structure and composition. A\ hether 
these intruded bodies of basic and acid material have ascended in one of 
the old volcanic funnels and have been injected laterally in laccolitic fashion 
has not been ascertained. Mr. Harker, indeed, is rather inclined to refer 
the intrusions to a time not only later than the Borrowdale volcanoes, but 
later even than the terrestrial movements that subsequently affected the 
district and gave the rocks their present cleaved and faulted structures. 
Besides the gabbro and granophyre of this locality, igneous activity has 
manifested itself in the uprise of numerous later dykes and veins, intei- 
mediate to basic in composition. Some of these are glassy (tachylyte) and 
spherulitic or variolitic.“ 
Throughout the Lake District a consideral)le number of bosses of more 
acid rocks rise through the Skiddaw Slates, and likewise through the volcanic 
group even up to its highest members. Some ol these bosses may possibly 
indicate the site of volcanic vents. Two of them, which form conspicuous 
features on either side of the Vale of St. John, consist of microgranite, and 
rise like great plugs through tlie Skiddaw Slates, as well as through the 
' Op. cit, p. 70. 
^ Mr. Harker, Quart. Jouni. Geol. Soc. vol. 1. (1894) p. -312, li. (1895) p. 125. (Jcol. Mag. 
1894, p. 551. 
