A hypersthene andesite from Drumyat (Ochils)

Abstract

The structure of the West End of the Ochils, where they terminate in the cliff overlooking Bridge of Allan and the Forth valley, is well exposed in many fine natural sections. Their extreme west end consists of massive beds of volcanic conglomerate, seen well in the narrow pathways behind Airthrey Castle. These are only irregularly bedded, and, while large blocks are few, they are, on the whole, coarse, and contain little fine detritus. The blocks have a rounded and distinctly water-worn appearance, and it is possible, every here and there, to find among them nests and patches of sand and shale, in a few places, amounting to distinct beds of sandstone. The general dip is to the N.W. about 10°. The lavas are strikingly limited in development in the western area, while, as we ascend the slopes of Dumyat, they gradually become more and more important, and, finally, in very great measure replace the fragmental rocks. In the precipitous face of the hill overlooking Menstrie and Logie Church, every bed of lava and ash can be well seen, as they follow one another in regular succession. They form a regularly interbedded series, the lavas thinning out gradually as they pass westwards, while the ash beds close over them—a feature suggesting their origin from a volcanic focus situated to the eastward or southeastward. On the level moor, which forms the back of Dumyat, the sections are not quite so clear, but the structure is not difficult to read. The Sherriffmuir Road,