404 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 6, 



greatly altered and is quite crystalline. In some cases their being 

 now visible is due to the nature of the surrounding material, which 

 is so slightly quartzose that their outline is well defined ; but in 

 other cases it is due to the character of the quartz of the grains -of 

 sand being different from that of the quartz subsequently developed 

 in the rock. Thus it often happens that, whilst the quartz of the 

 schist itself is clear and transparent, that of the grains of sand con- 

 tains many fluid- cavities and minute acicular crystals, and is milk- 

 white or brownish, so that, when seen with a AVenham's condenser, 

 each of the grains has a distinct outline, though surrounded by the 

 clear quartz which has been deposited on them in crystalline con- 

 tinuity. The grains of felspar-sand are in a similar manner sur- 

 rounded by quartz, near the junction containing granules due to the 

 decomposition of the felspar, which occasionally appears to have been 

 itself metamorphosed into a fine-grained mica-schist. Some of these 

 facts are seen to great advantage in the sericite slate (a kind of 

 green mica-schist), which occurs near AViesbaden in Germany, and 

 also in the mica-schist of the Highlands of Scotland ; and since 

 I first observed them last autumn, I have been surprised to find 

 how frequently they may be seen, when carefully looked for. I 

 scarcely need say that their occurrence in metamorphic rocks is 

 a very important fact, for they show most clearly that mica-schist 

 originally contained grains of sand, and that there has been a 

 subsequent crystallization of quartz throughout the whole rock ; 

 whilst other facts prove that there was a crystallization of mica, 

 garnets, felspar, and other minerals, so as to completely alter its 

 original character, and form what we call a metamorphic schist. It 

 appears to me that nothing establishes this more conclusively than 

 the structure of those varieties of mica-schist which possess what I 

 have described as " cleavage-foliation " in papers read before the 

 British Association (Report/Trans. Sects. 1855, p. 96, and 1856, p. 78). 

 I will especially call attention to a thin microscopical section which 

 I have lately prepared of a mica-schist from Muchuls, about ten 

 miles south of Aberdeen, which contains in itself proof of all the 

 leading facts connected with metamorphism. It was originally part 

 of a band of ripple-drift, and therefore the material must have been 

 deposited from a gentle current of water, but has since been com- 

 pletely altered into mica-schist with cleavage-foliation. In ripple- 

 drift in fine-grained sandstones of perfectly unaltered rocks, the 

 small stratula of sand are separated by layers of argillaceous matter, 

 and the bands of stratula are also separated by a similar deposit. 

 These facts readily explain the structure of the ripple-drift in mica- 

 schist ; for in those parts where in unaltered rocks we find sand or 

 impure clay, in mica- schist occur bands of quartz or mica, the 

 sandy parts corresponding to the quartzose, and the argillaceous to 

 the micaceous. But this is not all; for the stratula have been 

 greatly disturbed by mechanical forces, have been placed at angles 

 at which they could not possibly have been deposited, and have been 

 bent into contortions that show clearly the direction of the pressure, 

 as will be best understood from the following drawing : — 



