4. A rock-marl underlies the cornstone in the form of a reddish, variegated sandstone, and contains about fifteen per cent. of lime. Deep sections of this calcareo-arenaceous deposit are displayed along the basin of the Tay, on both sides, from the confluence of the Isla to Stanley, at Pitcairn Green on the Almond, and occupy the ridge from Methven to Crieff. A remarkable vein of serpentine skirts the base of the Grampians in a south-east and north-west direction, of a beautiful dark olive-green, in some places of a blue and whitish color, and at Cortachie Bridge, where it crosses the Esk, containing crystals of diallage. This dyke widens in some parts to nearly ninety feet, of a hard compact texture, and, as the marble of the district on the lakes of Clunie, it is extensively used for ornamental purposes. 5. The geologist, as he pursues his journey by either of the lines of railway that intersect Forfarshire, has still many interesting localities and objects before him. Traversing “the fertile plains of Gowrie” by the Perth and Dundee Junction, he enters at Inchture upon a higher member of the old red sandstone, a fine-grained yellow-spotted bed. The deposit first appears to the eastward of Inchture, in the den of Balruddery, where its outcrop is seen immediately to overlie the gray fossiliferous beds.—The same variety emerges on the opposite bank of the Tay at Birkhill; at Abernethy, where it abuts at nearly right angles against the trap in a small ravine to the south of the village; whence it skirts the base of the Ochils, and occupies the center of Strathearn at Dumbarnie. The Clash-bennie sandstone, doubly interesting from having furnished the first and best specimen of holoptychius, the type of its age, may be regarded as an extension of the Balruddery and Inchture rock. The beds vary a little in their lithological characters, as well as in the deep flesh-color predominant in the latter; still the spherical markings are there, and, as their organic remains are identical, their position in the series may be considered as one and the same. The yellow or upper beds of the old red sandstone fall next to be considered; but these, from their geographical limits, are deferred to the subsequent chapter.