438 ME. J. PARKINSON ON AN INTRUSION OF GRANITE [Aug. 1 899, 



part unstriped, in which stand semi-porphj-ritic plagioclases. The 

 felspar surrounding all is of late consolidation, and extends as a 

 kind of matrix for some distance among the adjacent crystals. On 

 one side may be seen the grains into which a neighbouring felspar 

 is in process of being resolved. 



Occasionally the corroding substance has been quartz. In some 

 of these instances, offshoots proceeding from a quartz-grain have 

 spread irregularly in the substance of the felspar, but the con- 

 tours which the latter mineral presents to the corroding one 

 are the same as those described above. At the same time, there 

 exists a tendency, not perhaps very strongly marked, for the 

 felspar to polarize as a mosaic rather than as a single individual. 

 There is, as in other cases, a granular arrangement of the kaolin. 

 It seems evident that this composite granular structure, often 

 seen in thin sections without the accompaniment of the clearer 

 felspathic substance, is a contact-phenomenon due to the mole- 

 cular rearrangement of the substance of the felspar under the 

 altered conditions attendant on the introduction of basic material. 

 The fact that these peculiarities are found beautifully developed in 

 large orthoclases derived from the granite, but now embedded in 

 dark and basic material (fig. 1, p. 436), points in the same direction, 

 namely, that a rearrangement of the particles of the felspar has 

 taken place without actual melting, and hence without the introduc- 

 tion of new and foreign material into the composition of the mineral 

 itself. Such a supposition would lead to the conclusion that the 

 aggregation of the kaolin into spots, together with the resolution 

 of the crystal between crossed nicols into a mosaic of grains, is an 

 early stage of a process which ultimately leads to the production 

 of ' eyes ' of one or more species of felspar embedded in a clear 

 uniform base, presumably of orthoclase. In a very few cases this 

 shows traces of microperthite. Occasionally, as in fig. 2, p. 436, the 

 appearance of the uniform matrix or base of felspar-substance con- 

 taining plagioclase-crystals, and appearing, at all events, to corrode 

 and split off parts of the kaolinized felspar lying in its neighbour- 

 hood, suggests invasion and corrosion by a felspar-substance, in the 

 main, no doubt, that of the granite, but probably containing also 

 material derived from the absorption of the plagioclases of the 

 diabase. The grains produced by the reconstitution of the felspar 

 usually appear to form equally well in all directions ; but in the- 

 case of the plagioclase-crystals there seems in some cases to have 

 been a formation of grains with greatest facility parallel to the 

 twinning-planes of the felspar, and, to some slight extent, at right 

 angles to this direction. The corroding quartz also has occasionally 

 acted along the direction of twinning. 



One noteworthy point remains, and that is, that in the not very 

 common case of a mica-flake embedded in the altered felspar, the 

 production of the granular structure in the latter has preceded the 

 formation of the former. 



One other peculiarity mentioned before (p. 433) is a structure 

 identical with that described and figured by Lacroix as ' groupe- 



