130 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 4, 



thick beds of slaty conglomerate, full of small quartz-pebbles. The 

 lowest member of the Old Red Sandstone consists of coarse ferru- 

 ginous sandstone, alternating with red marls, and containing some 

 very irregular beds of little thickness of nodular limestone mixed with 

 steatite, which have been worked by the side of the lake above Bal- 

 maha. This set of beds forms a low arch dipping to the south mider 

 a great thickness of coarse red conglomerate, and dipping also towards 

 the slate on the north, which has, no doubt, led to the limestone it 

 contains being erroneously mapped as part of the slate series. The 

 coarse conglomerate rises into a high ridge of hills, which terminates 

 at Balmaha. A similar ridge, formed of the same member of the 

 Old Red Sandstone, is a prominent feature a little to the south of the 

 slates, along a great part of the Highland Border ; it is everywhere 

 remarkable for the large size of its pebbles, often a foot in diameter, 

 which distinguish it from the beds both above and below it. Near 

 Balmaha this conglomerate dips E. 40°, S. 40°, and contains an- 

 other bed of limestone worked on the north face of the ridge. The 

 conglomerate is overlaid on the south by a red sandstone, dipping at 

 first 30°, and becoming more nearly horizontal to the southward. 



The limestone of Auchmar lies also in the lowest member of the 

 Old Red Sandstone, between beds of red sandstone and variegated 

 marl, overlaid by the coarse conglomerate. The bed of limestone 

 varies in thickness, nowhere exceeding 7 feet ; it is nodular and 

 mixed up with marl. Only the extreme rarity of lime in the High- 

 lands could cause so poor a bed to be worked. 



On the west side of Loch Lomond the clay-slate is quarried at 

 Luss, where it rises between two great bands of trap. The green 

 slate occurs on the north of the clay-slate, and rests on the mica- 

 schist ; here, as at Aberfoyle, overlapping and concealing the northern 

 edge of the dark clay-slate. Judging from MacCulloch's descriptions, 

 both the formations of slate are continued to the westward along the 

 north of the Firth of Clyde. 



At the first glance the positions of the slate in the sections just 

 described are perplexing, and appear contradictory. To account for 

 them, we must suppose that the trap-rock, which breaks through in 

 several places, is continued below the surface, and runs along the 

 whole line of the Highland Border. In some parts of its course it 

 has raised up the whole mass of the slate, until it all dips to the 

 north towards the mica-schists. In other parts, only the southern 

 half of the slate-series is so raised up by the trap, and the northern 

 portion dips southward, and rests in a less disturbed position on the 

 mica-schist. The manner in which the clay-slate is brought up alter- 

 nately on the north and south side of the green slate is very remark- 

 able, and can only be explained on the supposition that it is concealed 

 in some places by an overlap of the newer deposit of green slate. 



From Aberfoyle eastward the band of slate-rocks follows a nearly 

 regular course of about E. 30° N., with a slight deflection near Strath 

 Earn ; but to the west of Aberfoyle its course is less regular, being 

 successively thrown to the south in a series of breaks : the most im- 

 portant of these is connected with the elevation of Ben Lomond, where 



