426 Reviews — A. Daubree's Experimental Geology. 



general principle being to operate upon a glass tube placed inside 

 one of iron. The tubes were exposed to a temperature of about 

 400° C. during several weeks. It was found that pure water 

 suitably superheated will transform an anhydrous silicate, such as 

 glass, into a hydrated silicate of zeolitic character. A glass, for in- 

 stance, containing silica 68-4:, lime 12-0, magnesia 0'5, soda 14*7, 

 alumina 4 - 9, was found to lose silica, alkali, and alumina, whilst the 

 relative amount of lime seems to have increased, and the new silicate 

 had fixed a quantity of water. 1 



The results varied according to the different nature of the glass, 

 and of the substance added to the water. Crystals of quartz were 

 frequently formed. A figure is given at page 168 of a slice sub- 

 jected to polarized light, showing a number of colourless microliths, 

 and a few greenish crystals of pyroxene. The weight of the water 

 which effected all these altei'ations was only one-third of that of the 

 glass transformed. Obsidians and perlites were also tried with 

 partial siiccess : some crystals of felspar and pyroxene associated 

 with these underwent no alteration, but were " sugared " over with 

 little crystals of quartz. M. Daubree enlarges on the wonderful 

 results which have thus been obtained in the wet way, even to the 

 production of an anhydrous silicate, such as pyroxene, at tempera- 

 tures far below the fusing-point of the mineral. The application of 

 these results to the phenomena of metamorphism is obvious, and 

 explains, amongst other things, by the ready liberation of silica 

 from compound silicates, the formation of quartz on a large scale. 



The Roman masonry at Plombieres and elsewhere, so long in 

 contact with warm mineral springs, affords an admirable instance of 

 " contemporaneous metamorphism." The reaction of the alkaline 

 waters upon the mortar and bricks has resulted in the deposit of 

 chalcedony and of many zeolitic minerals. Microscopic sections of 

 bricks with geodes lined with chabasite, etc.. afford very effective 

 pictures, and many interesting chemical details are added. The 

 curious white concretionary substance called Plombierite — gelatinous 

 when moist, and transported and deposited by the water like silica 

 itself — appears, according to Fouque's analj'sis, to be a compound or 

 mixture of carbonate and silicate of lime with water. All these 

 results have been effected at moderate temperatures and without 

 pressure. A valuable digest of these facts appeared in the Quart. 

 Journ. Geol. Soc. for 1878 (vol. xxxiv.), where M. Daubree observes 

 that zeolitic minerals may be considered as a kind of " extract " of 

 the rocks that have been subjected to a continued lixiviation. 



Water, he therefore insists, has played an important part in the 

 crystallization both of eruptive and of metamorphic rocks, especially 

 as it permits the crystallization of silicates at temperatures far below 

 their fusing-points ; we must thus look to the hydrothermal rather 

 than to the dry way for the explanation of the origin of silicates in 

 a large part of the rocks. 



Under these circumstances, and adopting the very comprehensive 



1 Some of the. figures in the tables at page 161 appear to have got transposed, as 

 the calculations hardly agree. 



