146 Mr Le Hunte on the Geology of 



mixed with black slate, through which it has broken. The 

 mass of felspar, however, that may here be examined, as well as 

 that of the mixture, into which it graduates, is so great, as to 

 prevent any mistake in determining the nature of their connec- 

 tion. The upper part of the felspar, for a short distance, is 

 composed of rather large, sub-globular concretions ; and it is a 

 remarkable fact, that the pyrogenous rocks of this district have 

 often assumed the concretionary structure. The upper bed of 

 columnar felspar, at the interesting mountain Cader Idris, often 

 exhibits what may be called an incipient concretionary struc- 

 ture, giving it much the appearance of a sandstone, the grains 

 of which have been softened and agglutinated by heat. A fels- 

 pathic rock, generally compact, but occasionally granitoidal, ex- 

 tends from Carnarvon nearly to Bangor, where it becomes 

 coarsely granular, and resembles a sandstone. The passage 

 from the compact to the granular state, although gradual, is so 

 rapid, that it may be seen in a hand-specimen ; on discovering 

 this fact, T concluded that the felspar was derived from a sand- 

 stone by the agency of heat. To this conclusion I was, in 

 some degree, led, by what I had seen in Anglesea ; where there 

 is reason to believe, as Mr Henslow has observed in his geo- 

 logy of that island, that an imperfect granite, very similar to 

 that near Carnarvon, has been formed by the action of heat on 

 the old red sandstone. It appears more probable, however, 

 until further evidence be procured, that the apparent sandstone^ 

 near Bangor, is concretionary and derived from the felspar, 

 than that a ridge eight miles in length of the latter is derived 

 from the former, so completely changed by heat, that no trace 

 of its original character remains. This ridge extends westward 

 of Carnarvon, where it is formed of sienitic greenstone. Geo- 

 logists have discovered what they consider volcanic tufas among 

 the transition-rocks. Such tufas certainly exist in North Wales ; 

 and if the term may be applied to breccias, in which foreign 

 fragments are embedded in a volcanic cement, they occur there 

 more frequently than in any other part of Great Britain. In 

 some instances, the fragments appear to have been entangled in 

 the volcanic rock when forcing a passage to the surface ; in 

 others, there can be no doubt that the cement, although com- 

 pact, and containing both crystals and concretions of felsp 



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