328 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



large number of careful rock analyses, which show the rela- 

 tive admixture of the different rock-forming substances. 

 By careful chemical analyses, Robert Bunsen succeeded in 

 distinguishing between two volcanic magmas exuded from 

 different vents in Iceland the one, a normal trachytic or 

 acid magma, the other a normal pyroxenic or basic magma, 

 and showed that from the combination of these all possible 

 transitional varieties of eruptive rock might take origin. After 

 the publication of Bunsen's paper in Poggendorffs Annakn in 

 1851, geologists were so zealous in the chemical investigation 

 of rocks, that almost a thousand chemical and mechanical 

 analyses of rocks were forthcoming ten years later when Justus 

 Roth prepared his tabular list of rock analyses. 



In the year 1850, Henry Clifton Sorby published a short 

 communication on the Jurassic Calcareous grit, whose 

 structure he elucidated by applying Nicol's methods of ex- 

 amining thin rock -slices by transmitted light. In two further 

 treatises in 1853 and 1856 Sorby tried to solve the problem of 

 cleavage by similar means of examining thin sections of 

 cleaved rock. These earlier writings of Mr. Sorby were the 

 precursors of his famous memoir in 1860, which revolutionised 

 the teaching of petrography. Independently of Sorby, Oschatz 

 in Berlin had recognised the importance of preparing thin 

 slices of rock for microscopic examination. On the yth 

 January 1852, Oschatz exhibited a collection of fifty micro- 

 scopic slides of mineral sections at a meeting of the German 

 Geological Society, and again in 1854 at a Scientific Congress 

 in Gottingen, but he did not succeed in arousing any great 

 interest. 



The turning-point was Sorby's classic paper on the micro- 

 scopical structure of crystals, published in the Quarterly 

 Journal of the Geological Society in 1858. This paper demon- 

 strated the structure of rock-forming minerals with unprece- 

 dented accuracy; it compared the natural mineral crystals 

 with crystals artificially produced, and finally drew definite 

 conclusions regarding the origin of the different rocks. Sorby 

 was able to deduce from the presence of fluid, gaseous, 

 crystalline, vitreous, and slaggy inclusions in crystals, the 

 aqueous or volcanic origin of certain rocks, and thus brought 

 to an end questions which had been for many years matters 

 of dispute, and which could never have been solved without a 

 precise knowledge of the mineralogical elements and ground- 



