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de hi. 



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aiil 



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 els' 



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 flit 



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ALTERED CAMBRIAN ROCKS, LLANBERIS. 



107 



so much altered that they have been partly, and in some iippee 

 cases entirely, fused, and thus pass into true igneous rock. Gaiieey> 

 All the metamorphism of the Charnwood Forest rocks is WaU -case 45. 

 of this kind, nor do they ever assume the character of 

 gneissic rocks. It may be that dry heat, through the 

 intrusion of igneous rocks, was applied with too much 

 rapidity to allow of that slow chemical separation and re- 

 union of constituents, according to their affinities, in layers 



(often of stratification or 

 gneiss 



) which constitutes 



47.— Altered rock, serpentinous, with carbonate of 



lime and flaky layers of hornblende. Pierced by red crys- 

 talline felspathic veins. ~" 



Forest, 



Altered Cambrian grits and conglomerates, and quartz 



Map No. 78, S.E. 



and horizontal section No. 4, Wall 



48— Cambrian grit, Llyn Peris, Caernarvonshire, com- 

 posed of grains of quartz and felspar. 



^ Though excessively hardened, there is no igneous rock in 

 its immediate neighbourhood. The quantity of alkali in the 

 felspar renders such a rock peculiarly liable to alteration by 

 heat, and great alterations are found in similar rocks in the 



neighbourhood, when 



masses. 



in contact with errupted igneous 



felsp 



49.— Cambrian grit, similar to the above, but 



50. 



Felspar grains retain their crystalline form. 



more 





Altered Cambrian conglomerate from the north- 



Padarn 



This specimen is derived from a mass very near the quartz 



porphyry No. 52. It has originally been a conglomerate 



with a very felspathic gritty matrix similar to No. 49. 



ine base has been probably partially fused, the outlines of 

 so™ nf the pebbleg rendered indistinct) and the whole 



some of 

 hardened. 



