NICOL SOUTHERN GEAMPIANS. 199 



reversal of the beds on the north-west supposes. Second, the mica- 

 slate also shows that it has undergone an action conformable to this 

 supposition. The enormous twisting and contortion of the mica- 

 slate has long been considered as one of its most characteristic 

 features ; but it has been seldom observed that there is within these 

 beds a similar twisting and contortion of the minuter laminae or folia. 

 Not only are the beds, as such, bent and curved, but within these 

 beds a second set of contortions — convolutions, as it were, of another 

 order — have taken place. Xow any force acting from below would 

 evidently merely bend or curve the strata, and thus pull out or 

 lengthen their mass, and could not thus convolute or fold up the beds 

 on themselves. Such folds or convolutions imply a force acting on 

 the beds laterally, not vertically ; and not raising them up, but forcing 

 the ends or sides together*. Such a force, however, is exactly the 

 one required to fold the mica- slate over on itself and on the clay- 

 slate, and to raise up the red sandstone as seen on the south-east. 

 Were it necessary to assign an agent for this compression, I should 

 point, partly, to the great outbursts of granite in the central region 

 to the north-west, and, as even more influential, to the great com- 

 pression which the strata must have undergone when taken down 

 into those interior regions of the earth's crust where the chief labo- 

 ratories of metamorphic action are situated f. 



16. Central Gneiss and Quartette. — The next formation we have 

 to notice is the great central formation of gneiss. In this I include 

 most of the quartzite on its margin, as, although distinct in mineral 

 character, these two rocks in this region do not appear to me suffi- 

 ciently separate in other respects to be regarded as two independent 

 geological formations. This view was stated by me in the note to 

 my Geological Map of Scotland, and this central gneiss, with its asso- 

 ciated quartzite and limestone, referred to a newer formation, " not 

 older than the Lower Silurian period" j. At the Leeds Meeting of 

 the British Association in 1858, I also brought forward sections 

 proving that " in the central region of Scotland, from Aberdeenshire 

 to Argyllshire, the great formation of gneiss, with limestone and 

 quartz-rock, overlies the mica-slate, and does not dip under it as 

 usually represented. In particular, the gneiss of the Black Mount 

 and Breadalbane Highlands appears to form a wide synclinal trough 

 resting on both sides on mica-slate, and thus to be an overlying and 

 younger formation "§. Although these views differed widely from 



* This was long ago proved by Sir James Hall's simple experiment — all the 

 more demonstrative from its simplicity. 



f A similar view was proposed in reference to the section at the S.E. extre- 

 mity of the Grampians in my paper in Quart. Journ. Geol. Soc. vol. xi. p. 547. 

 Subsequently (Paper at Leeds Meeting of Brit. Assoc, Proceedings for 1858, 

 p. 96, and Xote on Geol. Map of Scotland) I was inclined to regard the clay-slate 

 as normally the lowest bed ; but the difficulties in the way of this supposition, and 

 further examination of the localities have caused me to revert to my former view. 



X Note on Geol. Map of Scotland, pp. 2 and 4. 



§ See Report of the Twenty-eighth Meeting of the British Association, held at 

 Leeds, 1858, Transactions of the Sections, p. 96 ; also Report of the Twenty- 

 ninth Meeting, held at Aberdeen, 1859, Transactions of the Sections, p. 119. 



