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In the Geological Survey's Memoir in explanation of Sheet 76 (p. 21) occurs the statement—“Small intrusive masses of pinkish felsite and white felstone occur at Windyfield and on the slope of Ord Hill, near Rhynie.” Early in 1910 two small specimens were collected by the author from an exposure of the former rock on the north side of the road leading from the village of the Muir of Rhynie in Aberdeenshire westward to Cabrach in Banffshire, at a point about half a mile from the former place. Both of these were at the time believed to be fairly typical specimens of the “intrusive felsite” above referred to. These specimens were not examined microscopically till sometime in the summer of 1911. Subsequently the examination of more typical specimens showed one of them to be a peculiar rock of a gritty character, much infiltrated with cherty material, and its vesicles to be due to the weathering out of some highly soluble original mineral—that it was, in fact, a highly specialised rock of sedimentary origin. The other specimen was found on section to be a fine grit of a buff colour containing numerous volcanic fragments in addition to composite fragments of both acid and basic plutonic rocks in a fine quartzo-felspathic matrix. Some of the included fragments of volcanic rocks showed numerous scattered felspar microlites, but usually without definite orientation as in a typical volcanic rock. Others contained porphyritic felspars and quartz crystals of some size. These included fragments were almost uniformly of
William Mackie (Thu,) studied this question.