1888.] MICROSCOPICAL JOURNAL. 73 



so far shown the conditions under which many of the rock structures are pro- 

 duced, that we are often able to state the conditions under which a given rock 

 solidified from the microscopic examination alone. Those rock structures 

 which from a study of modern volcanic action w^e are led to believe are the 

 I'esult of consolidation at or near the surface of earth have been yielded by 

 artificial magmas, which were subjected to conditions of more or less rapid 

 radiation. The granitic structure has never been produced artificially, and 

 the conditions of occurrence of rocks in nature which possess this structure, 

 are such as to leave little room for doubt that their formation took place far 

 below the earth's surface, where radiation of heat was extremely slow. 



The more the crust of the earth is studied, the more forcibly is it shown 

 that the rock masses of which it is composed are constantly changing. In 

 some parts, generally below the surface, the rocks are becoming more crys- 

 talline from the action of heat and pressure ; while at the surface alteration 

 of minerals is taking place, yielding more soluble substances, of which the hy- 

 drous minerals and the carbonates are the most common. The earth's crust 

 is, in fact, a huge laboratory in which reactions are constantly taking place, 

 some of which we have thus far been unable to reproduce. This is, proba- 

 bly, owing to the impossibility of introducing into our experiments the proper 

 conditions, particularly those of time and uniformity of temperature. From 

 these changes in the composition of rocks, we learn that each mineral, in any 

 given rock-mass, possesses a more or less complicated life-history, which is, 

 in many cases, comparable for variety of change with the life-history of some 

 of the organized plants and animals. These life-histories form a special field 

 for the microscope to explore, and the brilliant efforts of Lossen, Tornbohm, 

 Hawes, and a number of others, have shown what may be realized in this 

 direction. 



No attempt should be made to mention some of the more important attain- 

 ments in the science of petrography without reference to the work of Hermann 

 Vogelsang. At about the time that Zirkel undertook the microscopic study 

 of minerals, Vogelsang began his investigations on the phenomenon of crys- 

 tallization. He was able to retard the process of crystallization by mixing the 

 solution with Canada balsam. Microscopic bodies, more elementary than 

 crystals, were discovered and carefully studied, to which he gave the name 

 of crystallites. These bodies were found to fuse together in the formation of 

 crystals. Vogelsang's work entitled Die Krystalliten^ which was published 

 by Zirkel* after the death of its author, I'emains a monument to his ingenuity 

 and carefulness. 



The importance of the results obtained in the microscopic study of rocks, 

 and the increasing number of workers in that field, warranted the publication 

 of two text-books in 1873, which were intended to embody the most impor- 

 tant results that had been realized. One of these was the work of Zirkel and 

 was entitled. Die mik?-oskopische Bcscha^enheit der Mhiej'alien zi?zd Ges- 

 teine. The author of the other was Heinrich Rosenbusch, who is to-day the 

 acknowledged leader in the science of petrography. His work was entitled, 

 Die mikroskopische Physiographic der petrographisch wichtigen Miner- 

 alien. In 1877 he published a second volume entitled, Die mikroskopische 

 Physiographic der massigen Gesteine. These two volumes have been 

 revised and largely rewritten, the first appearing in its new form in 1885, and 

 the second in 1S8S. Aside from the ordinary scope of a text-book, they con- 

 tain a complete record of obtained results previous to the date of publication. 



The researches of Fouque and Michel-Levy, and others, on the composition 

 and structure of artificial magmas ; of Boricky and Behrends in developing 



♦Bonn, 1875. 



