1863.] SORBY MICA-SCHIST. 403 



generally be seen very distinctly. The existence of this " ripple- 

 drift " appears to me to prove most conclusively the sedimentary 

 origin of the rock in which it occurs. Its various peculiarities are 

 so characteristic that it could not, I think, be confounded with any 

 structure known in any unstratified rock; and therefore, since I 

 find that it is by no means rare in the mica-schist of the Highlands 

 of Scotland, I contend that it is a most convincing proof of the 

 material having been originally deposited from water, whilst other 

 facts prove quite as conclusively that the present schistose condition 

 was produced by subsequent crystallization. That these conclusions 

 are also borne out by the relations of the rocks on a large scale is 

 shown in the interesting paper by Murchison and Geikie, published in 

 the Society's Journal, vol. xvii. p. 232. The localities in which I have 

 more especially met with this ripple-drift are in the neighbourhood of 

 Arroquhar and of Aberdeen. Along the coast between the latter 

 place and Stonehaven I have seen some most excellent examples 

 with all the characteristic peculiarities, but the rocks are often so 

 much contorted that very complicated appearances have been pro- 

 duced. Such structures are also very well seen in a small quarry 

 between Arroquhar and Tarbat ; and were it not that the most com- 

 plicated cases gradually pass into others sufficiently simple, one might 

 sometimes be induced to abandon all hope of a satisfactory explana- 

 tion. I must here remark that it is often almost impossible to 

 make out the true structure by examining the rock in its natural 

 state. It should be ground down to a smooth, flat surface, and 

 slightly varnished, when the structure is revealed in a most striking 

 manner. The contortion and compression of the beds have produced 

 very various and curious results. In some cases the small stratula 

 have been greatly contorted, whilst the oblique beds have been but 

 little affected. In others the latter have been much disturbed, and 

 the stratula forced together, so as to be nearly all parallel ; but so 

 various and complicated are the relations, that possibly I cannot 

 explain them better than by saying that nearly all the results are 

 met with that could be produced by bending and squeezing bands of 

 ripple- drift in all sorts of positions. 



Though specimens ground flat and varnished show the general 

 structure to very great advantage, yet many, more minute par- 

 ticulars can only be learned by preparing thin transparent sections 

 for microscopical examination, — some facts being best seen by using 

 a Wenham's parabolic condenser, and others by reflected or by 

 transmitted light, with or without polarizing apparatus. When the 

 coarser varieties of clay- slate are thus examined, the separate grains 

 of sand, composed of quartz, felspar, or of compound rocks, can be dis- 

 tinctly seen ; but very commonly no such grains are visible in the 

 typical varieties of mica-schist. In them the quartz exists as masses 

 of smaller or larger crystals, not bounded by crystalline planes, 

 but mutually interfering, so as to give rise to a very different struc- 

 ture from that of quartzose slates. However, I have lately met with 

 many specimens of typical mica-schist in which the original grains 

 of sand may be distinctly seen, even although the rock has been 



