Zeolitic and Siliceous Incrustations. 235 



in Amygdaloid and other altered Volcanic Rocks." By Prof. A. 

 Daubree, F.M.G.S. 



The author described the formation of zeolitic minerals by the 

 infiltration of masonry by the waters of thermal springs at Plom- 

 bieres (Yosges), Luxeuil (Haute-Saone), Bourbonne (Haiite-Marne), 

 and near Oran in Algeria. In breaking through the wall of concrete, 

 composed of fragments of stone and brick built by the Romans 

 round the mouths of these springs, it has been found that the 

 materials employed have undergone a great change by the long- 

 continued action of the water. The cavities in the bricks are occu- 

 pied by minerals, generally zeolitic, among the most abundant of 

 which is chabasite, agreeing in all respects with the natural mineral. 

 Phillipsite or lime harmotome also occurs, associated with the pre- 

 ceding, just as in the amygdaloidal trap of Iceland. In hollows of 

 the calcareous cement small crystals of apophyllite occur, with pul- 

 verulent and minutely crystalline fluor spar, together with other 

 minerals not identified, but resembling in character gismondine and 

 scolezite. At Plombieres hyalite occurs with the zeolites ; and where 

 the masonry is exposed to the full flow of the water there is a 

 transparent gelatinous deposit which becomes white and opaque 

 when dry, and is a hydrated sihcate of lime analogous to okenite. 

 Arragonite occurs generally in acicular crystals, but sometimes of 

 the form called apotome by Haiiy, found in iron-ore deposits and in 

 some basalts. Calcite is frequently associated with the chabasite, 

 as in Icelandic lavas. Halloysite is also met with at Plombieres. 



Besides the formation of geodes in the visible cavities, the whole 

 substance of the bricks was found to be altered by contact with the 

 water. This change rendered the originally friable brick very hard 

 and compact ; and microscopic examination showed that its minute 

 pores were filled with colourless and transparent mineral substances. 

 The author gave the following list of the minerals thus found — 

 chabasite, Christianite (?), mesotype, hyalite (?), tridymite, chalce- 

 dony in radiate spherules, calcite, and some globular bodies of un- 

 certain nature. The association of chalcedonic quartz with opal is 

 interesting, as proving that silica maybe deposited in the anhydrous 

 form when the temperature of the surrounding medium does not ex- 

 ceed 70° C. Analysis showed that the amount of zeolitic sub- 

 stance added to the bricks is from 13 to 14 per cent, of the total 

 weight. 



This contemporary production of zeolites and other minerals 

 identical with those found in amygdaloidal and many other altered 

 volcanic rocks is regarded by the author as an experimental demon- 

 stration of the mode of formation of the latter, which are no doubt 

 produced in a similar manner by the percolation of water through 

 the substance of the rocks, conveying to, and depositing in, their 

 cavities mineral substances dissolved during its passage. Zeolitic 

 minerals, as he says, may therefore be considered a kind of " ex- 

 tract" of the rocks so subjected to continued lixiviation. And 

 the process being effected independently of any peculiar conditions 

 of heat and pressure, would seem to show that no such conditions 

 arc essential in the production of natural zeolites. 



