Microscopic Characters of Devitrified Glass. 



87 



"On the Microscopic Characters of some Specimens of Devitri- 

 fied Glass, with Notes on certain analogous Structures in 

 Rocks." By Douglas Herman and Frank Rtjtley. Com- 

 municated by Professor T. Gr. Bonne y, D.Sc., F.R.S. 

 Received May 28, 1685. Read June 18. 



(Plates 1—4.) 



Devitrification is a process which may either take place natural Iy 

 or be brought about by artificial means. Instances of the former are 

 familiar to as in once glassy rocks which have passed into a felsitio 

 or micro- crystalline-granular condition. The ohange which has taken 

 place in the conversion of ohsidian into felstone is so great that it 

 would not be possible to infer the original nature of the rock, were it 

 not that certain structural peculiarities, often of a very delicate cha- 

 racter, are retained. It is, indeed, a most remarkable feature in such 

 rocks that a physical change so complete should fail to obliterate the 

 perlitic structure and the fine streaky markings or fluxion- bands 

 which are common in the vitreous lavas of every age. The microscopic 

 recognition of such structures has of late years added considerably to 

 our knowledge of the rhyolitic rocks and tuffs of Archaean and 

 Paleeozoic times, many of which were undoubtedly hyaline rhyolites. 

 Through devitrification their original character has been obscured, 

 and in many instances can only be revealed by the -use of the 

 microscope. Although much has been written upon this subject, 

 including Vogelsang's admirable work,* we are still in compara- 

 tive ignorance of the conditions under which such devitrification 

 has taken place. The experiments of Daubree,,f made upon glass 

 tubes at a high temperature and pressure, in presence of water, 

 resulted in the development of a schistose or foliated structure, 

 corresponding with the cylindrical form of the tubes acted upon : 

 the development in some cases of reticulating cracks, due to contrac- 

 tion, the transformation <of the glass into a friable substance with a 

 structure both fibrous and concentric, and also into a hard material 

 with similar structure. Professor Daubree has likewise in these 

 experiments produced .radiating crystalline or spherulitic bodies, 

 microliths, and actual crystals ^of pyroxene and quartz. The glass 

 upon which he operated contained, in its .unaltered condition — 



* " Die Krystalliten." Bonn, 1875. 



f " Etudes Sj.nthetiques de vGeolcgie iExperimentale," ,p. 155. Paris, 1879. 



