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THE BASIC MASSIVE ROCKS, ETC.
443

little diallage. Their separation from the gabbros and the hypersthenites seems to be upon mineralogical grounds solely; since emphasis is laid upon the fact that their feldspar is apparently anorthite. Of such great importance was the mineral constitution of rocks regarded at this time, that we find no statement made with respect to the similarity between many diabases and many gabbros. The facts pointed out by earlier investigators to the effect that augite and diallage are but slightly different varieties of the same mineral, had been overlooked, or had, at any rate, been regarded as of little importance, since these expressions of opinion had for the most part not been founded on the study of thin sections. The microscope was used principally for the determination of the nature of the constituents of rocks, and had therefore emphasized their mineralogical composition out of due proportion to its importance.

The influence of Zirkel's book upon geologists in all parts of Europe was soon felt in the increased number of purely petrographical papers published in the journals; and this increased interest soon manifested itself in studies that included more than a mere description of rock sections. Vogelsang[1] had, years before, shown that there were great possibilities in the new science of petrography, but in the flush of excitement over the discovery of an easy and exact method of rock analysis, these possibilities were left unexplored until geologists became quite well acquainted with the essential components of the most important rock types.

Soon after the composition of the important rock types became fixed, attention was turned more particularly to their structure. Professor Judd[2] examined the gabbros in the denuded cores of Tertiary volcanoes in Scotland, and found that while diallage is the prominent pyroxene of the lower portions of the

  1. ↑ H. Vogelsang: Philosophie der Geologie und Mikroskopische Gesteinsstudien. Bonn, 1867.
  2. ↑ J. W. Judd: The Secondary Rocks of Scotland.Second Paper. On the Ancient Volcanoes of the Highlands and the Relations of their Products to the Mesozoic Strata.Quart. Jour. Geol. Soc., XXX. 1874, p. 220.
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