I have reserved for a separate description some interesting circumstances relating to Glen Tilt, because they are extraneous to the general appearances, which required to be considered in one collective point of view, and because they would have interrupted the continuity of the narrative. The first of these is a deposition of calcareous matter, which was pointed out to me by the Duke of Atholl, to whose unwearied kindness and friendship I am no less indebted for the facility which I experienced in examining this district, than the science of geology is for this very interesting fact. On the brow of the hill above Gow's bridge a large bed of shell marle is to be seen. It is found on the left side of the river commencing under a face of limestone and extending for about 250 feet or more down the hill. It is between 80 and 90 feet wide, and its depth seems to vary from one foot to six or more. This appearance is unexpected but it is not solitary, as another similar instance occurs at the foot of the limestone ridge at the south of Ben Gloe, as well as on the hill of Tulloch formerly mentioned, but in neither of these cases so well marked or so extensive as the present.
The formation of shell marle in the peat mosses of Scotland is I believe well known to the Society, as it has long been to the agriculturists of Scotland. These deposits are by no means uncommon, whether in the larger tracts of peat, or in the smaller ones which are found occupying insulated patches on those hollows in the mountains which are favourable to the growth of this substance. They are found forming beds, generally of no great thickness, but varying from an inch or two to as many feet, and lying under the
