70 W. M. Hutchings — Clays, Slates, and Contact- MetamorpMsm. 



of muscovite, to be converted into biotite by direct combinations 

 with those minerals we cannot say. Probably some geologists would 

 not doubt the power of intense pressure to bring this about. We 

 may question whether pressure alone could do this, or anything like 

 it, but when high temperature and water are also considered, acting 

 at the same time, we must at least look on this method of formation 

 as not outside the limits of consideration. 



(3) Newly -formed quartz and felspar. — We cannot well consider 

 the occurrence of these " regenerated " minerals separately, because 

 though they may not occur together, yet when they do so occur 

 their modes of formation appear to be intimately connected. 



The quartz is the more frequent, and, even when felspar occurs, is 

 the more " sensitive," occuring at earlier stages and being often largely 

 developed before any felspar is seen. 



It seems obvious that the quartz-grain of the "contact-mosaic" 

 bears no direct relationship to the clastic grain of the original 

 slates, etc. The latter, with its more or less angular outlines, its 

 fluid-cavities, and its practical freedom from enclosures, has dis- 

 appeared, and in its place we have the round, or in the best-developed 

 mosaics the polygonal, grain, free from fluid cavities, and containing 

 usually numerous microlites, biotite crystals and grains, and other 

 enclosures, these grains of quartz fitting to one another and to the 

 other constituents of the mosaic quite closely and exactl3^ 



It is here still less possible to see how this change can be due to 

 any simple re-formation of each individual grain of quartz, — to its 

 mere recrystallization in situ. Neither is it due to any simple 

 addition to its outer portions of any freshly-deposited silica. 

 Looking at these mosaics carefull3% it does not seem easy to come 

 to any other conclusion than that the quartz in them, as a whole, 

 has recrystallized out of some sort of solution in which were, 

 forming or already formed, the microlites and other bodies which 

 are now enclosed within the quartz grains. 



Many fine-grained slates do not contain any distinguishable grains 

 of quartz at all, and in these, as also in slates where such grains are 

 plentiful, there is a good deal of exceedingly finely-divided quartz 

 or other forms of free silica. It is equally difficult to see how this 

 could be formed up into the larger grains of the mosaic, and charged 

 with enclosures, without a previous re-solution and recrystallization. 



Taking now the felspar, where such occurs, all the same con- 

 siderations apply. The grains in the mosaic have the same charac- 

 teristics as regards general outlines, enclosures, etc., and it is clear 

 that the quartz and felspar were crystallized together, though there is 

 a distinctly greater tendency in the felspar to develop idiomorphic 

 forms.' 



1 It should be noted that the " tesselated " or " mosaic " structure is by no means 

 peculiar to the alteration of sedimentary rocks. At Shap, for instance, it is very 

 beautifully developed in some of the altered rhyolitic and andesitic ashes in the 

 contact-zone ; sometimes it is quartz alone, sometimes quartz and felspar, and some- 

 times felspar alone which forms these mosaics. 



A tesselated structure has been looked upon by some observers as quite specially a 

 result of dynamic metamorphism, but this idea cau, of course, no longer be enter- 



