296 MESSRS. A. HARKER AND J. E. MARR ON 



pleochroism, between purplish-brown and colourless. Apatite 

 occurs very rarely in little veins of qunrtz and mica, in a fashion 

 which seems to indicate a mctamorphic origin [798J. 



The above-mentioned minerals collectively make up a consider- 

 able part of the metamorphosed andesite. The remainder of the 

 rock consists of a finely granular groundmass, the precise nature of 

 which is less easily studied. A portion of it is quartz, but careful 

 scrutiny detects here and there in the grains the evidence of 

 twinning and even of twin-la mellation. It is doubtful in some 

 cases how much of the original felspar of the andesites is preserved 

 as such in the less metamorphosed examples. The process of recon- 

 struction is seen, however, in some of the occasional porphyritic 

 felspars. One of these will be found to be studded with little 

 flakes of brown mica and partly transformed into a granular 

 aggregate, M'hile enough of the original felspar-substance remains to 

 vaguely indicate the twinning between crossed nicols [799]. In 

 the vicinity of the granite, the whole substance of the rock is 

 certainly transformed, and the granular aggregate in which the 

 coloured minerals are embedded assumes the perfectly clear ap- 

 pearance so well seen in many " crystalline schists." The twinning 

 of the granules can be verified only occasionallj^, although it is 

 evident from chemical considerations that a considerable proportion 

 of the aggregate must consist of felspar. These highly altered 

 rocks share with many of the products of dynamo-mctamorphism 

 their singular immunity from subsequent secondary changes, a 

 property Avhich seems to require some physical explanation. It is 

 curious to compare in a junction-slice of altered andesite and 

 granite the fresh minerals of the former with the turbid felspars 

 and discoloured micas of the latter. 



The contents of the vesicles and of certain narrow cracks posterior 

 to the filling of the vesicles have undergone instructive transforma- 

 tions. It is here that the first effects of the metamorphic agent were 

 manifested. The silica is found to have recrystallized in a mosaic 

 of clear quartz-grains, free from fluid-cavities. In the less meta- 

 morphosed examples this process is incomplete, the central portion 

 retaining its confused, almost cryptocrystalline, structure. In the 

 highly altered rocks the change to a rather coarsely-granular mosaic 

 is universal, and this at its outer boundary is not very sharply 

 separated from the surrounding rock. 



The delessite is here almost constantly replaced by the green 

 hornblende described above, the brown mica occurring but rarely 

 either in the vesicles or in the occasional narrow veins which 

 represent cracks in the rock [798]. The vesicles contain horn- 

 blende even when the surrounding rock is densely charged with 

 flakes of mica (see PL XI. fig. 5). The hornblende is well- 

 cleaved, and sometimes the whole or a large part of the mineral 

 within one vesicle is in crystalline continuity. One slide [897] 

 shows one of the narrow veins alluded to running straight through 

 the section. It contains clear quartz or, in some parts of its length, 

 hornblende. The vein traverses two vesicles, and in each case the 



