348 LTEUT.-GEN. C. A. M c MAHON OK ROCKS OF [A-Ug. l894 T 



scales has been profusely formed in the matrix, and also in the 

 felspars, producing, in some cases, the appearance of their having 

 been corroded by the groundmass. Dots of magnetite are arranged 

 in flowing lines, and there is a general appearance of fluxion- 

 structure, especially in No. 26. It does not seem necessary, how- 

 ever, to assume that any extensive movement of the ash actually took 

 place. These fluxion-lines may only indicate that the fine volcanic 

 dust was arranged in flowing lines of lamination round the larger 

 fragments, and gave a direction to the heated aqueous agencies 

 that subsequently acted on the rock. On the other hand, the 

 very fine dust that must have formed the original interstitial 

 portion of these ashes, when subjected to the aquose heat radiating 

 from the great masses of Dartmoor granite may have become 

 sufficiently plastic to assume a fluxion -structure under the stress of 

 some of those minor earth-movements, or tremors, that were 

 doubtless abundant in this region during the time when the granite 

 was cooling down. 



No. 26 must have been at one time in a plastic condition and 

 must have been subjected to some pressure, because in one place an 

 elongated felspar is bent round an included fragment, and cracked 

 transversely in several places. But the amount of movement 

 and shearing must have been slight, because several sharp angular 

 fragments and felspars stand up boldly at right angles to the 

 lines of apparent fluxion, and the felspars have not become eye- 

 shaped, but, on the contrary, retain the sharp-broken, fragmentary 

 outlines which they received when blown out of the mouth of the 

 crater. Most of the cracks in these felspar-fragments are probably 

 due to the explosion which ejected them from the crater ; but the 

 rock was, I conceive, subsequently subjected to partial aqueous- 

 fusion and compression. I regard this specimen as a very beautiful 

 example of the way in which a pyroclastic rock may, under the 

 influence of powerful contact-metamorphism, be made to assume 

 the appearance of a lava. 



Near the railway viaduct some of the agglomeratic beds contain 

 quite large blocks of slaty and felspathic rocks, of all sizes and 

 shapes. I have examined a fragment from one of these blocks : — 



No. 28, 1160. This is a fragment of a felsite. The matrix is 

 microcrystalline-granular and contains porphyritic crystals of 

 felspar and quartz, with dots of magnetite, ferrite, and leucoxene. 

 These crystals have suffered corrosion from the base. 



This slice is chiefly interesting, because the production of 

 granophyric structure in one of the felspars appears to be directly 

 connected with a quartz-vein that runs up to it. This vein is not 

 an aqueous infiltration along a crack : the margins are not 

 straight, well-defined lines ; and the quartz appears to have eaten 

 into the matrix on either side of the vein. This case supports the 

 opinion expressed in describing No. 16 (p. 315), that granophyric 

 structure in some cases, at all events, is due to secondary alteration. 



In this connexion it may be desirable to allude to a vein that, at 



