Yol. 55.] GEANITE AN'D DIABASE AT SOREL POINT. 437 



corroded and lies half embedded in orthoclase. A few long 

 brownish felspars have no doubt been derived from the diabase. 

 The quartz, late in formation, plays the part of a groundmass, the 

 patches isolated in ordinary light often showing optical continuity 

 between crossed nicols over large areas ; but the structure is not 

 granophyric. Occasionally irregular tongues of a felspathic substance 

 penetrate the felspars in a way which shows that some change has 

 taken place in the latter since their first formation. In ordinary light 

 we see that there is a strong tendency on the part of the kaolin of the 

 felspars to gather into rounded spots producing a patchy appearance, 

 generally more apparent in one part of the mineral than in another. 

 Accompanying this, an irregular polarization, more or less pro- 

 nounced and foreshadowing the approach of a granular structure, 

 becomes visible between crossed nicols in the part of the felspar 

 nearest to the region of the later felspathic substance, and seems to 

 carry the process of change some way ahead of this invading material, 

 if such it be. As the process of breaking up becomes further 

 advanced, the component granules — before only hinted at — assume 

 definite outlines, generally very faint in ordinary light, and finally 

 «tand as complete individuals, granular or possessing more or less 

 definite edges, in an uniform felspathic base (fig. 1). This process 

 of reconstitution may go through all its stages in a single crystal. 

 Viewing such an one in ordinary light, the gradual aggregation of 

 the kaolin into spots in passing away from the unaltered part is 

 clearly seen, and is shown also, on slowly rotating the stage 

 between crossed nicols, by slight differences in the positions of 

 extinction in various parts of the crystal. Where the granules 

 stand out singly or in groups in the felspathic base, the latter 

 •embays the altering crystal in a manner most clearly seen between 

 ■crossed nicols, making it appear as though it had invaded the 

 original felspar, splitting off portions as it advanced. 



The same structure is found in other slides, and is yet more clearly 

 displayed when more basic material is present. The granular 

 individuals, when well developed, polarize separately as units ; 

 though occasionally adjacent grains polarize together. They show 

 sometimes plagioclase-striping, and now and then a zonal structure 

 is seen on slowly rotating the stage. The uniform felspathic base 

 in which these granules are set may either behave uniformly or 

 may break up between crossed nicols into more or less clearly- 

 defined areas, thus making the structure as a whole poecilitic. 

 When best developed, this arrangement appears clear and free from 

 %aolin. It seems evident that these peculiarities in the felspars are 

 due in some way to the action of the absorbed basic rock, which has 

 caused part at least o£ the orthoclase entirely to change its con- 

 stitution. Occasionally the corroded crystal is a plagioclase (see 

 fig. 2),^ surrounded by an uniform felspar-substance, for the most 



^ Fig. 2 shows multiple twinning at 6 as a continuation of the twinning of 

 the felspar. In the felspathic substance a are embedded numerous smaller 

 individuals. The two spaces c are not in continuity with a, but show a different 

 polarization. The granules at d represent the breaking up of a felspar-crystal 

 in the manner described in the text. 



