,^1 



POIiPHYRITIC TRAP. 13S 



craters to be formed by a partial sinking down of the summits, when 

 the mountains were still softened by heat. It may, however, deserve 

 the future enquiry of geologists, whether the red felspathic trachyte on 

 one side of the crater of High Stile, which forms Red Pike, and 

 extends over the mountain, may not at once have flowed as lava. 



Many mountains in Cumberland and Westmoreland are composed 

 of porphyritic trap, passing into clinkstone. In a deep ravine of 

 Swarthfeil in Cumberland, opposite the seat of J. Marshall, Esq. 

 the mountain, which is here composed of clinkstone, presents the 

 columnar structure on a magnificent scale ; the columns are slightly 

 bent and inclined. 



Porphyry, from an intermixture with hornblende, frequently pass- 

 es into sienite ; when this is the case, the latter rock generally forms 

 the upper part of the mass. Porphyry and basalt, in enormous 

 masses, often cover the primary mountains in the Andes. According 

 to Humboldt, " they are arranged in regular columns, which strike 

 the eye of the traveller like immense casdes lifted into the sky." 

 Some geolo2;ists describe four formations of porphyry ; but this divis- 

 ion is purely theoretical, as those who admit it, agree that the diffe- 

 rent formations of porphyry frequently pass into each other ; and, 

 from the evident connection of porphyry and basaltic with igneous 

 rocks, it naturally follows, that such transitions must take place. 

 Many porphyritic rocks may be regarded as more ancient than ba- 

 saltic rocks, as porphyry frequently occurs intermixed with, or cov- 

 ering, transition rocks, and basalt is most commonly associated with 

 the secondary strata. 1 am informed by Professor Sedgwick, that 

 the porphyry of the Cheviot Hills has produced frequent and great 

 dislocations of the beds in its vicinity. We have few instances, be- 

 side, that I am acquainted with, in England or Wales, of eruptions 

 of well defined porphyry : they are not uncommon in Scodand and 

 in the Alps. We shall proceed to describe the phenomena present- 

 ed by trap rocks, of which there are numerous striking examples in 

 Great Britain and Ireland. 



In describing the phenomena presented by any of the trap rocks, 

 we describe those peculiar to every member of the trap family. 

 Were it allowed to express a geological fact in familiar terms, it might 

 be said, that all the members of this family give indications of a fiery 

 character, and of having been troublesome neighbors to the adjacent 

 rocks, disturbing them, and even changing their nature, when they 

 are closely associated. Beside occurring in overlying unconform- 

 able masses, all trap rocks, with porphyry, which may be placed at 

 their head, are occasionally found intersecting other rocks like verti- 

 cal walls. It has been before stated, that these vertical walls are 

 called dykes^ — the term dyke and wall being synonymous in North 

 Britain. The substance which most commonly occurs in dykes is 

 basalt ; and as these basaltic dykes are well known, from their fre- 

 quently intersecting coal strata, we shall now give a description of 

 basaltic dykes, and their effects on the adjacent rocks or strata. 



