420 S, ALLPOET ON THE METABEOEPHTC EOCKS 



angular outline, and several of them present the characteristic forms 

 of aggregations of magnetite. Many have the central parts occupied 

 by granular patches of that substance, which appear to be corroded 

 round the edges, and pass gradually into the brown matter in such a 

 way as to indicate that the latter is the product of their alteration. 

 They must, I think, be regarded as pseudomorphs after magnetite ; 

 but I have not been able to determine their composition. 



Their mode of occurrence is precisely that of magnetite, being 

 found in the crystalline constituents and in the matrix ; in the latter 

 they are surrounded by innumerable minute flakes of brown horn- 

 blende or mica, while that at a short distance is usually green — a 

 fact clearly indicating the diffusion of ferric oxide. 



In the rocks just described some of the original constituents are of 

 considerable size, and have partially preserved their former character 

 and structure; but in more compact varieties the minute consti- 

 tuents have been so completely decomposed and obliterated as to be 

 quite unrecognizable in their present condition. 



A specimen of this kind occurred in the mass of rock near Tolcarn, 

 one part of a large block being coarsely crystalline, the other fine- 

 grained — a circumstance of common occurrence in rocks of this 

 character. 



In a thin slice of the compact portion the most conspicuous con- 

 stituent is actinolite, with its radiating blades and flakes either fill- 

 ing veins or thickly disseminated through the mass in small patches, 

 many of which were evidently cavities. These are everywhere sur- 

 rounded by an extremely fine granular matrix containing many 

 comparatively clear spaces. Augite, in small grains, was abundant, 

 and has been converted into a brown granular substance. 



The metamorphism is here so complete that there are no distinct 

 traces of any crystalline forms ; it is, however, quite possible that 

 such forms may never have existed ; for there are compact basaltic 

 rocks of various geological periods having a minute granular or 

 crypto-crystalline texture ; and as regards the felspar, a rock from 

 Clicker Tor (to be presently described) and other examples show 

 clearly that the crystalline form is frequently quite obliterated. 



An examination of this specimen only would not have enabled me 

 to determine positively its igneous origin ; I had seen, however, that 

 it formed part of a crystalline mass of unquestionable character ; and 

 the following examples not only afford a striking illustration of the 

 various degrees of metamorphism to which these rocks have been 

 subjected, but also exhibit every gradation from an obscure speci- 

 men like the one just described, to others readily capable of recog- 

 nition. 



A specimen taken from an old quarry in Eose Hill, near Castle 

 Horneck, is a highly altered dolerite intermediate in texture between 

 the coarse and compact varieties just mentioned. The remains of 

 the felspar prisms are still traceable ; and there is the same minutely 

 granular texture ; there are many grains of magnetite and of altered 

 granular augite, together with much flocculent green matter, but 

 very little clearly recognizable hornblende. The slice is traversed 



