74 PROF. A. DAUBREE ON INCRUSTATIONS BY THERMAL 



glass tube. Among the other substances of the same nature are 

 various minerals, which have not been perfectly identified — the 

 principal ones being small globules studded with microscopic crystals, 

 which are apparently gismondine, a mineral found under similar 

 conditions in the scoriaceous lavas of Capo di Bove near Eome, and 

 crystalline needles having the appearance and chemical characters of 

 scolecite. 



At Plombieres the zeolites are accompanied by translucent and 

 colourless opal or hyalite in considerable quantity ; and at the same 

 place a gelatinous deposit was found upon parts of the masonry re- 

 ceiving the full flow of the hot water, which was also transparent and 

 colourless when collected, but became opaque and snow-white in dry- 

 ing, forming mamillated masses of a fibrous structure like other con- 

 cretionary minerals, which melted with intumescence before the blow- 

 pipe, and gelatinized with acids. This is a hydrated silicate of lime 

 without alkali, analogous to okenite, a mineral found in amygdaloidal 

 rocks in Ferro, Iceland, and Greenland. It has not been possible, 

 however, to determine its composition exactly, owing to the fact of its 

 being intimately mixed with opal and hydrated silica. 



Aragonite is occasionally seen in very acute double six-sided 

 pyramids of the form called apotome by Haiiy, with the face e x , and 

 resembling the crystals found in iron-ore deposits, and in some 

 basalts ; but it is more commonly found in acicular crystals. 



Calcite occurs in many of the cavities associated with chabasite, 

 as in the Iceland lavas. The crystals are of many forms ; both 

 scalenohedra with rhombohedral terminations (d&) and rhombo- 

 hedra have been observed. In some white or occasionally violet- 

 coloured deposits obtained at Plombieres I have found fluor-spar 

 both in the pulverulent form and in microscopic crystals. These are 

 often found in the neighbourhood of apophyllite crystals which also 

 contain fluorine. 



In addition to the above minerals the hydrated silicate of alumina 

 known as halloysite has been found at Plombieres, the principal seat 

 of this mineral being, however, in the fluor-spar veins from which 

 the hot springs rise. 



In addition to the geodes of crystals visible to the naked eye, the 

 paste or substance of the bricks has undergone a change, the nature 

 of which has been determined both by the aid of the microscope 

 and by chemical analysis. This change however, is, apparent at 

 sight when a brick which has undergone alteration is compared with 

 others of the same period that have not been subjected to the action 

 of the spring. While the latter are generally imperfectly burnt and 

 friable, those included in the concrete are often hard, very compact, 

 and sonorous, recalling the texture of phonolite, and break into thin 

 acicular and trenchant fragments under the hammer. 



The microscopic examination of these bricks shows that their 

 substance is intimately impregnated with different minerals even at 

 points entirely removed from the infiltration visible to the naked 

 eye. For the purpose of studying these changes, transparent sections 

 of an unaltered brick were prepared as terms of comparison, the 



