332 HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



series of closely-related modifications which could be optically 

 distinguished. 



In addition to the service rendered by microscopic methods 

 in facilitating the accurate mineralogical identification of 

 the chief constituents of rocks, these methods disclosed the 

 existence of a considerable number of subordinate mineral 

 constituents which had either been wholly overlooked by 

 macroscopic research or had been supposed to be extremely 

 rare. 



To mention one example, augite was found to be present in 

 granite, porphyry, rhyolite, "phonolite, etc. This discovery was 

 a direct contradiction to previous teaching that certain minerals 

 could not exist in association with each other in the same 

 rock; amongst other couples quartz and augite, orthoclase and 

 augite, leucite and plagioclase, had been said to be mutually 

 exclusive. 



Microscopical research made it possible for the first time 

 to attain a clear conception of the different kinds of rock- 

 structure. The composition and structure of the ground-mass 

 in hemicrystalline rocks was revealed, and new light was 

 thrown upon characteristic structures of glassy rocks, fluxion 

 structure, spherulitic and perlitic structure. Hence with the 

 aid of the microscope the origin of the crystalline rocks began 

 to be better understood, and their relations to the group of 

 apparently homogeneous rocks. 



The indifference with which the large body of geologists 

 had long viewed the microscopic study of rocks now gave 

 place to zealous interest, and from the year 1870 the very 

 large number of special papers that were devoted to 

 petrological subjects not only filled Mineralogical Journals, 

 but occupied a large share of the space in the Journals of 

 Geological Societies. The sudden influx of new literature was 

 unprecedented, and it would be hopeless to attempt to men- 

 tion individual papers in the present work. Between 1870 and 

 1880, two-thirds of the publications on microscopic petro- 

 graphy belonged to Germany and Austria. Amongst British 

 workers Allport, Rutley, Houghton, Bonney, Archibald Geikie, 

 Teall, Harker, may be named ; in North America some of the 

 pioneers were A. Hague, Whitman Cross, Iddings, G. H. 

 Williams, Wadsworth, Lawson. 



The results of these researches necessitated many changes in 

 the systematic arrangement of the rocks, and in no group was 



