THE BASIC MASSIVE ROCKS, ETC. 443 



little diallage. Their separation from the gabbros and the hyp- 

 ersthenites 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 constitu- 

 tion 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 pet- 

 rographical 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 im- 

 portant 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 : Philosophic der Geologie und Mikroskopische Gesteins- 

 studien. 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 Mes- 

 ozoic Strata. Quart. Jour. Geol. Soc, XXX. 1874, p. 220. 



