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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



submitted to you the interesting results of his own researches upon 

 the devitrification of artificial glasses and slags ; and the subject 

 has since been pursued by Yelain in Prance, and by Hermann and 

 Eutley in this country. 



The igneous rocks supply us with admirable opportunities for 

 studying Mineral Embryology. In the same rock-mass we may 

 sometimes find every possible gradation, from an almost perfect glass 

 to a noncrystalline aggregate. By the study with the microscope 

 of the several transitions in different parts of the mass, we obtain 

 data for the most important conclusions concerning the phenomena 

 of crystal-development. 



There is another line of research in connexion with Mineral- 

 Embryology, which appears to be full of promise, and which has not 

 yet received all the attention it deserves. In the " contact-zones " 

 around great igneous intrusions, we find the curious so-called 

 " spotted slates," which under the microscope are seen to contain 

 nebulous patches, the mere ghostly presentments of crystals, strug- 

 gling into being in the amorphous mass. The development of these 

 nebulous masses into perfect crystals, exhibiting the characteristic 

 external forms and optical properties of andalusite and cyanite, of 

 garnet and epidote, of hornblende and mica, may be traced in some 

 cases with the greatest facility. 



More complicated still are the phenomena exhibited along the 

 foliation-planes of the rocks, which have been made to flow in the 

 act of mountain-making. There, as the old minerals are destroyed, 

 new ones build themselves up from their elements. The study of 

 all the steps of this process is an undoubtedly difficult one ; but the 

 results already obtained by Eeusch, Lossen, Heim, and Lehmann, by 

 Lapworth, Teall, Boland Irving, and Williams, lead us to look hope- 

 fully forward to the full solution of the grand but complicated 

 problems of regional metamorphism. 



The field of Mineral-Embryology is indeed a promising one, and 

 its diligent cultivators may hope to gather a harvest no less rich 

 than that which has been reaped by the workers in the same depart- 

 ment of the Biological Sciences. 



Let us now turn from the statical aspect of minerals, their 

 Morphology, to the dynamical aspects, their Physiology. 



Minerals are not fixed and unchangeable entities, as they are 

 sometimes considered. On the contrary, they exhibit varying 

 degrees of instability, and pass through very definite series of 

 metamorphoses. 



