NICOL SOUTHERN GRAMPIANS. 193 



Are these planes of deposition and bedding, or a true slaty cleavage * 

 distinct from, and independent of the stratification ? Or are theso 

 two structures here coincident ? This question is of very consider- 

 able interest in relation to the true structure of this locality, and of 

 the Highlands generally. It has too often been assumed, without 

 proof, that all the marked division-lines in the metamorphic strata 

 of the Scottish Highlands are lines of deposition, and much error 

 and confusion has thus been produced. Near Comrie, this question 

 is not easily decided, as the rocks, chiefly exposed in natural sections, 

 or quarries of small extent, furnish few precise data for its determi- 

 nation. Still even there the great regularity in these planes, and the 

 intimate manner in which they pervade the entire mass of the rock, 

 correspond more to cleavage than bedding. Other planes of struc- 

 ture, too, may be observed in these rocks. One set of these show a 

 very predominant dip west by compass, or "W. 25° to 30° S., and 

 thus with a strike to N. 30° W., almost at right angles to the direc- 

 tion usually assigned to the strata in this region. This dip is very 

 clearly seen in the coarse greywackes at the foot of Loch Earn, and 

 also in the contorted mica-slate at St. Fillans Church. The latter 

 rock consists of thin, undulating laminae, dipping N. 27° W., but 

 intersected by other planes dipping W. 27° S. parallel to the bedding 

 of the slates. Similar planes dipping generally W.S.W. are obser- 

 vable in the whole valley from Loch Earn to Comrie, but near the 

 latter place the dip is more nearly west. In the valley of the Led- 

 nock a second set of division-planes also occur, but far less regular in 

 position. 



The slate -quarries wrought in the Aberuchil Hills, in Glen Artney, 

 to the south-west of Comrie, offer better evidence on this question. 

 These slates are of a light bluish-grey colour, with a fine granular, 

 almost compact texture, occasionally becoming fibrous or minutely 

 crystalline. The cleavage is very distinct, so that the slates appear 

 scaly, or as if made up of very flat and thin lenticular plates. The 

 cleavage is cut obliquely by other planes, some very distinct, others 

 little more than mere striae, or lighter and darker lines. Where 

 very numerous and close, these give the slates a glossy, fibrous aspect. 

 The more distinct lines are readily traced in hand-specimens, and 

 appear as great curves on the exposed faces of rock in the quarry. 

 They do not intersect the cleavage at any fixed angle, but range 

 from 15° to 45°, and 60° in some instances which I measured. These 

 curved lines seem to me to represent the planes of deposition, whilst 

 the slates are wrought on the true slaty cleavage. The dip of the 

 cleavage is IS". 12° to 27° W., thus parallel to the foliation of the 



* It has been affirmed that true slaty cleavage does not exist in these meta- 

 morphic strata. Mr. D. Sharpe had great merit in directing attention to its occur- 

 rence in many parts of the slates, though in some points going further than I can 

 find good reason for following. (See Quart. Journ. Geol. Soc. vol. viii. p. 126, and 

 Phil. Trans. 1852, p. 445.) In the Quart. Journ. Geol. Soc. vol. xv. p. 113, I 

 described this structure in the slates of Easdale and Oban, where a marked 

 parallelism in the cleavage is combined with remarkable convolutions in the 

 beds. Compare, for cleavage in the Aberdeenshire slates, Eep. Brit. Assoc, for 

 1859, p. 118. 



VOL. XIX. PART I. O 



