lx PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April IQI4, 



him to complete his course there, and he was glad to accept an 

 offer from a rich South American, who wanted a tutor for his two 

 sons. In this capacity Rosenbusch passed about five years of 

 his life at Bahia; in Brazil. 



This part of his life is only imperfectly known, and in particular 

 we are not informed how he was led to exchange literary for 

 scientific interests. When he returned to Germany, it was to 

 study science at Freiburg and Heidelberg, and he received his 

 doctorate from the former University in 1868, at the late age of 

 thirty- two. At that time the application of the microscope to the 

 study of rocks was in its infancy. Zirkel, inspired by Sorby, had 

 led the way, and had already proved some of the capabilities of 

 this line of attack. Kosenbusch was attracted to the new study. 

 His inaugural dissertation was on the nephelinite of Katzenbuckel, 

 near Heidelberg ; and in the enthusiastic preface by which it is 

 introduced we can read the language of a man who is conscious of 

 having found his life-work. 



The pioneers in this line encountered difficulties which, thanks 

 to their labours, their successors can scarcely realize. The minerals 

 in a rock-slice, presenting quite new appearances, had to be recog- 

 nized by new tests. The application of the polariscope to this end 

 had been indeed one of Sorby's own contributions, but in practice 

 it was employed at first only in a general way. Mineralogists had, 

 however, examined the optical properties of numerous minerals in 

 sections cut in determinate directions. Rosenbusch collected these 

 data, supplemented them by observations of his own, and showed 

 how they could be systematically applied to the identification of 

 crystals cut in random directions in a rock-slice. In this way he 

 laid the foundation of microscopical petrography as an exact science. 

 His results were published in 1873 in the ' Mikroskopische Physio- 

 graphic der petrographisch wichtigen Mineralien,' which was 

 followed four years later by the companion volume on igneous 

 rocks. 



In 1873 also he was appointed Professor Extraordinarius at 

 Strassburg, and for some years he was attached to the Geological 

 Survey of Alsace-Lorraine. The chief fruit of his work here was 

 the well-known memoir on the Steigerschiefer and their meta- 

 morphism by the granites of the Vosges. In 1878 he was chosen 

 for the chair of Mineralogy and Petrography in the University of 

 Heidelberg, and began that career as a teacher which he pursued 

 so long and so brilliantly. His laboratory became the resort of 



