IxXViii PEOCEEDIN-GS OF THE GEOLOGICAL SOCIETY. 



Another investigation, of gTeat interest, is that of the quartzife- 

 roTis porphyries as compared with recent qnartziferous eruptive rocks. 

 The analogy of the microscopic structure of the two series is said to 

 be complete, admitting that in the older one a molecular change 

 has taken place. The newer volcanic rocks of trachytic character, 

 including those of Java, of Campigha, the Euganean Hills, and the 

 rhyolites of Hungary, exhibit in theii' imbedded grains or crystals 

 of quartz numerous glass-cavities, testifying to the once fluid condi- 

 tion of the magma from -vrhich they were enclosed, whilst the older 

 porphyries are frequently found to contain, in part, similar glass>a 

 cavities, and in part, or sometimes exclusively, cavities filled more 

 or less with fluid. The Delft Professor infers that, in these latter 

 hollows, the glassy material has been in process of time decomposed 

 and dissolved out by the percolation of water, and that the porphy- 

 ries were solidified from a similarly fused magma, although even 

 this paste may have been modified from the glassy condition by 

 slow molecular change. 



That the mineral olivine plays a j)art in the augitic rocks analo- 

 gous to that of quartz in the porphyries is confirmed by examina- 

 tion of specimens (figured in the series of ten beautiful plates) 

 from Yesuvius and the Siebengebirge ; and their numerous glass- 

 cavities point to a similar genetic origin. 



The novel and elegant researches of the geological microscopists 

 form a valuable set off- as against the dicta of some of the bolder 

 experimentalists who would deny to nature the power of doing more 

 than they can themselves accomplish in their laboratories, and who, 

 protesting against the possibility of sundry crystallized minerals being 

 produced by fusion, are driven to wild and arbitrary inventions to 

 account for what we see in the Tertiaiy and modern lavas. 



When a sedimentary origin is gravely proposed for basalts and 

 elvan porphyries, geologists know far too well the incompatibility of 

 observed facts with such a proposition to be shaken in their previous 

 convictions ; but an examination of the microscopic enclosures of 

 the plutonic rocks farther confirms the conclusions of those ob- 

 servers who have examined the seats of modem volcanic action. 



jS'ot only may we cite, with full assui^ance, a list of minerals 

 produced in a crystallized condition from a melted mass, certain 

 kinds associated with certain other kinds in wonderful family like- 

 ness of grouping, at points of eruption widely distiibuted over the 

 globe, but even the higher temperature required for sublimation 

 may with confidence be occasionally called iu to explain the pre- 

 sence of some of the crystallized miueral species. The origin of the 

 countless crystals of specular iron (oligist) sparkling around a crater 

 or in the hocca of a lateral eruption, can be ascribed to nothing else 

 than the sublimation of the metal as a chloride; and, recently, 

 that high authority' Gustav Eose has shown that crystals of augite 

 have been formed by a similar process. Herr von Eath discovered 

 in the irregular fissures of a ciuder cone (the great Eiterkopf, near 

 Andernach) crystals of specular iron dotted with minute yellow 

 crystals, which proved to be augite ; and the conclusion drawn from 



