ORIGIN OF CRYSTALLINE ROCKS. 47 



a neutral saline containing seven or eight thousandths of mineral matters, chiefly 

 sulphates and chlorids of alkalies, and of lime and magnesia ; while that of Plombières 

 contains only about three ten-thousandths, and is also said to be neutral. As remarked by 

 Daubrée, it is probable that the action of the water in the formation of these mineral 

 silicates is, to a great extent, independent of its composition, since pure water, in acting 

 upon finely divided alkaliferous materials, soon becomes itself alkaline. 



As regards other silicated deposits from thermal waters, we may notice the case 

 of the baths of St. Honoré (Nièvre), the waters of which, having a temperature of 31° C, 

 yield a finely laminated white translucent substance in concentric layers, which 

 appears from analysis to be a hydrous silicate of alumina, with a large excess of silica, but 

 is probably a mixture. Mention is also made of a similar deposit from a mineral spring 

 at Cauterets, which is talcose in aspect, and according to qualitative analysis, is a 

 silicate of alumina, with magnesia and alkalies."" In this connection mention should be 

 made of the occurrence at the thermal spring of Olette (Pyrennes Orientales), of a crystal- 

 line silicate, having, according to Descloizeaux, the crystalline form of stilbite, of which it 

 has also the composition."*" 



§ 94. As an example of a zeolite, apparently in process of formation, may be mentioned 

 the observations of R. Hermann, who found in the crevices of a columnar basalt at Stol- 

 penau, in Saxony, an amorphous white plastic substance, which after some time changed 

 into acicular crystals of scolecite."" More recently, Renevier has described the occurrence 

 of a white subtranslucent matter, unctuous to the touch, gelatinous at first, but becoming 

 a plastic mass, and called by the quarrymen mineral lard, found in constructing a tunnel 

 in the molasse or tertiary sandstone near Lausanne, in Switzerland, in 1876. This substance, 

 which formed layers of from one to three centimetres on the walls of fissures, was said by 

 observers to have, in some cases, taken on a crystalline form, a fact, however, which 

 Renevier was not able to verify. When dried at 100" C, it was found to be a hydrated 

 double alviminous silicate, giving the oxygen-ratios of chabazite, 1 : 3 : 8 : 6 ; the bases 

 being lime and potash, with 3.14 per cent, of magnesia.*^ 



§ 95. A remarkable fact in the history of zeolites is that lately made known by the 

 researches of Murray and Renard, that a decomposition of volcanic detrital material, goes on 

 at low temperatures in the depths of the ocean, transforming basic silicates, " represented 

 by volcanic glasses such as hyalomelane and tachylite," into a crystalline zeolite on the 

 one hand, and the characteristic red clay of deep-sea deposits on the other. To quote the 

 language of the authors, this process, " in spite of the temperature approximating to 0° 

 C, gives rise, as an ultimate product, to clearly crystallized minerals, which may 

 be considered the most remarkable products of the chemical action of the sea iTpon the 

 volcanic matters undergoing decomposition. These microscopic crystals are zeolites, lying 

 free in the deposit, and are met with in greatest abundance, in the typical red-clay areas of 

 the central Pacific. They are simple, twinned, or spheroidal groups, which scarcely exceed 

 half a millimetre in diameter. The crystallograjjhic and chemical study of them shows 



" For a summary of the observations of Daubrée, the details of which are found in several papers, see his 

 Géologie Expérimentale, 1879, pp. 179-207. 



*" Cited by Dana, System of Mineralogy, 5tli Ed., p. 443. 



*' Jour, fur Prakt. Chemie, Ixxii. Cited by Dana, System of Mineralogy, suh voce Scolecite. 



62 



'* Bull, de la Soc. Vaudoiso des Sci. Naturelles, xvi, 15. 



