ANNIVERSARY ADDRESS OP THE PRESIDENT. IO3 



minute constituents. The mica crystals, while they exhibit a slight 

 tendency to a banded arrangement, lie about in all directions in the 

 bands, and the same is true of the other minerals. There is some 

 free quartz, but the granules are small and do not appear to be 

 very abundant. Another specimen, taken from rather nearer the 

 granite, exhibits a similar structure ; but the mica flakes are slightly 

 larger, so are the quartz granules, and the spots of andalusite are 

 a little more clearly defined, and are yet more distinctly bordered 

 by the mica. The process, in fact, of mineral segregation and 

 crystal growth appears to have been slightly protracted. Another 

 specimen, taken from rather nearer an exposure of granite, does not 

 materially differ, except that it exhibits some bands comparatively 

 free from andalusite, and consisting mainly of quartz and mica, A 

 specimen taken within a yard of the granite contains but little 

 andalusite, and consists mainly of quartz and mica with a few 

 garnets. As, however, a portion of the slide resembles the others, 

 I presume the mineral differences here are due to original compo- 

 sition (which, indeed, I inferred from the appearance of the rock in 

 the field) ; hence the main distinction is, as before, the more perfect 

 separation and insulation of the constituents. Lastly, in an actual 

 contact-specimen, the mica flakes are yet larger, the andalusite, in 

 parts, though not invariably, is rather more clearly defined, and there 

 are some irregular spots of a mineral resembling staurolite *. The 

 inferences which would be naturally drawn from the above description 

 are confirmed by numerous other specimens which I have exa- 

 mined t- The process of change (which of course depends, in the 

 first instance, on the original constitution of the rock) is somewhat 

 as follows : — A very considerable proportion of the constituents are 

 rendered capable of entering into new combinations. Suppose, then, 

 the rock was originally (as probably in very many cases) a fine- 

 grained mud or silt, consisting of kaolin, of felspar debris still 

 retaining the alkaline constituents, of very minute quartz granules, 

 perhaps with fragments of mica and more or less carbonaceous and 

 ferruginous matter. Heat and pressure, in the presence of water, 

 as shown by Daubree's experiments, tend to separate silica from a 

 silicate, so as to form free quartz and silicates with a lower proportion 

 of silica. The alkaline constituents combine, in the one case, with 

 some of theiron and the requisite silicates to form a potash-iron mica: 

 in the other, they combine to produce a paragonite. Subsilicates 

 of alumina are formed, such as andalusite, and the quartz set free 

 crystallizes, sometimes perhaps independently, sometimes, no doubt, 

 around original constituents as nuclei. jSTow this process of segre- 

 gation is in many respects analogous to that of crystallization in 

 certain of the igneous rocks, and I have observed that these quartz 

 granules have commonly the rather rounded but slightly irregular 

 outline which is characteristic of the felstone and some micro- 



* Perhaps the higher temperature has allowed the iron to enter into combi- 

 nation with the alumina silicate. 



t I have been kindly permitted to examine a fine series collected by Mr, All- 

 port, and now in the British Museum, from Cornwall aud other localities, and 

 another still in his own possession, 



t2 



