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I. Introduction.
Thirty-one years ago, Mr. George Barrow (1) presented to this Society his classic account of the intrusion phenomena of a group of ‘Older Granites’, in the Forfarshire area of the South-Eastern Highlands. That paper is distinguished by reason of the clear enunciation of the two outstanding conclusions to which the author was led. In the one was illustrated for the first time the effect of crustal stresses on the differentiation-course of igneous magma, a process which plays so important a part in present-day theories of petrogenesis; in the other, also for the first time, was indicated the subdivision of a group of highly-metamorphosed sediments into metamorphic zones characterized by the presence of critical index-minerals. So far as I am aware, his paper represents the first attempt in petrological literature to bring precision to the study of regional metamorphism, by laying upon a map zonal lines indicative of varying grades of metamorphism.
It is chiefly upon this aspect of Barrow's work that I shall dwell in the matter now set forth.
Nineteen years later (2) he extended the area of his zonal metamorphic map, at the same time increasing the number of zones. The map presented to the Geologists' Association thus covers the ground included in the quarter-inch sheet (No. 12) of the Geological Survey map of Scotland, extending to the Highland Border from Stonehaven to Dunkeld. In the meantime, other Scottish investigators by extended surveys of Perthshire and Argyllshire were bringing to light new data on the metamorphism
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