816 H. HICKS ON THE METAMORPHIC AND 



Auchnasheen in which the rocks are beautifully exposed, and, after 

 obtaining this clue to the arrangement of the rocks in Glyn Docherty, 

 I soon became convinced that the whole of the mountain consists of 

 the lower gneiss rocks, being altogether a highly crystalline series, 

 chiefly gneiss with garnets, mica-schists also with an abundance of 

 garnets, and some hornblende-schist. 



I now noted the strike carefully as I ascended to a height of be- 

 tween 2000 and 3000 feet, and found it to vary only from the points 

 N.W. to N., and hence generally in accordance with the gneiss of 

 the west, and not, as has usually been supposed to be the case in 

 the central Highlands, with a strike of from N.E. to S.W., as in the. 

 newer unaltered rocks. 



I found very few dykes amongst these rocks of Ben Fyn, and none 

 of any great importance. 



The crystallization is in no way local, but affects the whole 

 series equally ; and it is that form so peculiarly characteristic of 

 the older or pre-Cambrian rocks wherever they are exposed in this 

 country. 



The gneiss and other pre-Cambrian rocks in Anglesey, Caernarvon- 

 shire, and Pembrokeshire, especially the Dimetian or gneiss series, 

 appear to be much in the same state of alteration ; and the general 

 petrological resemblance of the latter to some of these gneiss rocks of 

 the Western Highlands will probably enable the two series to be cor- 

 related some day, or at least show that they belong to the same 

 geological epoch or horizon in the history of our globe. These 

 mountains of gneiss to the north of Auchnasheen attain a height 

 of over 3000 feet, and are entirely devoid of any traces of the un- 

 altered Silurian rocks found in Glyn Docherty, Glyn Laggan, &c. 



The question as to how the Silurian rocks could have been brought 

 into the positions indicated in the latter part of the section is a difficult 

 one. Is it the result of a natural overlapping of the older rocks east- 

 ward by these newer rocks ? or have they been brought into this posi- 

 tion by faults ? I saw no evidence of faults of sufficient magnitude to 

 bring them to this position alone ; but minor faults there are un- 

 doubtedly, and thesehave in some cases considerably altered the position 

 of some of the beds. On the whole, however, the evidence seems to 

 show that these Silurian beds have been deposited on the eroded edges 

 of the lower gneiss rocks nearly in the positions in which they are now 

 found, the present inclination of the beds being dependent upon sub- 

 sequent movements, accompanied by some faults. These upper beds 

 are undoubtedly made out of such materials as would be derived from 

 rocks similar to those which now underlie them and to which they are 

 unconformable ; and hence it is that sometimes a superficial exami- 

 nation may possibly lead one to associate them. The persistent and 

 equal metamorphism, the contorted character, and high N.W. strike 

 of the lower series is so marked, however, that for any distance it 

 seems almost impossible to confound them in any way with the 

 comparatively undisturbed and unequally altered beds of the upper 

 series, which also almost invariably strike in the opposite direction, 

 or from N.E. to S.W., at a low angle of dip. 



