DISTRIBUTION OF DYKES. 335 



which they penetrate by reduced pressure in a plane of intrusion is 

 still an open question. Dykes are formed during changes of level 

 of land, and are exposed at the surface by denudation. 



Ben Nevis. The abundance and variety of felspar porphyry in 

 great masses on the summit of Ben Nevis and in the valley of Glen- 

 coe are familiar to every traveller in the Highlands. The porphyry of 

 Ben Nevis was shown by Von Oeynhausen and Von Dechen to have 

 been erupted through the granitic basis of that mountain. The por- 

 phyries along the vertical precipices of Glencoe send veins through 

 the subjacent granites, in number proportioned to the proximity of 

 the situation to the great mass of porphyry. This rock varies through 

 every stage, from claystone to felspar porphyry, the transition being 

 sometimes gradual and sometimes sudden. Agglomerate composed 

 of fragments of claystones and porphyries like those on Ben Nevis, 

 and some in Cumberland, are often seen in Glencoe. 



Ben Cruachan. In the mountain of Cruachan, which overlooks 

 Loch Awe, the hornblendic granite and schist rocks are traversed by 

 a great variety of large felstone and porphyry dikes, and some changes 

 of appearance happen to the mica schist. MacCulloch l describes the 

 porphyry dykes as perpendicular, 

 varying from 3 to 50 feet in breadth, 

 traversing alike the schist and the 

 granite veins, but not intermingling 

 with either. Dykes of porphyry, of 

 different kinds and colours, may run 

 near or in contact with each other ; 

 but in all cases these and other 

 dykes of basalt or porphyry are very 

 distinct at the edges, though firmly 



united to the rock which encloses them. Fig. 68 shows veins of 

 granite traversing the schist of Cruachan, themselves crossed by dykes 

 of two kinds of porphyry. 2 



Cumbrian Mountains. In the Cumbrian mountains felstone por- 

 phyries occur in many situations, and with a great diversity of 

 character. Some have a basis of translucent grey or green felspar, 

 and included crystals of glassy felspar and quartz ; others are com- 

 posed of a red, opaque, granular felspar basis, and red felspar and 

 quartz crystals, as in rhyolites ; the basis of others is compact felspar 

 or hornstone, and some have a dark andesitic base, with small white 

 opaque felspar crystals. Most of them, like the amygdaloids and 

 dolerites of the same region, occur in overlying masses, as well as 

 dykes. They seem to have a geographical dependence of a peculiar 

 kind on the foci of granitic eruption. They are not abundant in or 

 very near to the granite of Wastdale, Skiddaw, or Shap, but occur at 

 small distances from each of those masses. The Valley of St. John 

 shows pale red felspar porphyry overlying slate, well crystallised red 

 porphyry in Armboth Fell, and various kinds of felspathic rocks under 

 Helvellyn. Dykes of variable greenish porphyry divide the slates of 

 1 Geol. Trans., iv. 2 Ibid., iv., pi. vi. 



