Vol. 65.] ANNIVERSARY ADDRESS OF THE PRESIDENT. llii 



Honoured Foreign Members, a man known and beloved by all — 

 Ferdinand Zirkel — was led to build the superstructure of modern 

 Petrography on the foundations which Sorby had laid. It was in 

 1862 that Sorby, when travelling with his mother up the lihine, 

 made the acquaintance of Zirkel, then a student in the mining- 

 department of Prussia. They visited together the Eifel, the Sieben- 

 gebirge, and the lake of Laach, and during their excursions they 

 eagerly discussed the nature of the volcanic rocks, their mineral 

 composition, and their intimate structure as revealed by the micro- 

 scope. Sorby described with enthusiasm the wonderful results of 

 his own studies, and after the day's work the conversation was 

 continued into the night. Finally, on returning to Bonn, Zirkel 

 was shown under the microscope some of the slides which Sorby 

 had prepared with his own hands. To Zirkel it was a revelation, 

 and from that time onwards he devoted himself enthusiastically to 

 the development of the new Petrography. Thus was the torch 

 handed on ! 



It would be a mistake to suppose that Sorby contented himself 

 with mere pioneering expeditions into unknown fields. His work 

 so far as it went was thorough, and his theoretical views were 

 always in advance of his work. Had it been otherwise, it would 

 not have proved so fruitful. Scarcely any of the ideas which are 

 now agitating the minds of petrologists would have proved altogether 

 strange to Sorby. Consider his views on the order of consolidation 

 in igneous rocks : how modern is this ! The magma contains 

 minerals in solution, as water contains salts ; on cooling they are 

 deposited, and crystallize out at a temperature heloiv that of their 

 fusion-point when fused alone I As an analogy he cites the case of 

 aqueous solutions, when very infusible salts crystallize out after 

 the very fusible water, and may be obtained growing upon crystals 

 of ice. This view, published in 1858, stands quite apart from 

 another, perhaps better known, in which he attributes a certain 

 amount of water to the composition of the magma itself, as in the 

 case of granite : a conclusion which seems to follow from his study 

 of quartz-veins. 



The seductive problem of allotropy, in which Nature seems to be 

 on the verge of revealing the secrets of crystalline structure, could 

 not fail to appeal to the mind of Sorby ; and the more speculative 

 side of his genius, still allied however with experiment, appears in 

 a series of papers on the tetramorphism of carbon. These contain 

 some anticipations of views expressed by Mr. Barlow and myself. 



