82 PROF. A. DAUBREE ON INCRUSTATIONS BY THERMAL 



pletely dissimilar chemical constitution is furnished by the pheno- 

 mena witnessed at Bourbonne, where the zeolites have been deposited 

 side by side with fahlerz (tetrahedrite), copper pyrites, and other 

 usual constituents of metalliferous veins, by one and the same water 

 at one and the same temperature, according to the nature of the 

 material presented to its action. A further point of resemblance is 

 furnished by the crystals of calcite accompanying the zeolites in the 

 deposits of the springs, which affect varieties of forms exactly 

 analogous to those observed in mineral veins. 



Among the more remarkable localities where zeolites have been 

 found, is the lignite of Kerguelen's Island, according to the analysis 

 given by Percy*. This lignite is known to be of Tertiary age, and 

 is associated with volcanic rocks. 



In conclusion, the argument presented in the preceding pages may 

 be summarized as follows : — The production of zeolites and their 

 associated minerals by the action of thermal springs is not confined to 

 the formation of crystals lining the visible cavities of the bricks, but 

 has extended to the compacter portions, the same minerals being found 

 in the smallest cavities, when examined by hand-lens or microscope, 

 which latter method of investigation had not been applied previously 

 to the present communication. The bricks so altered, present many 

 remarkable points of resemblance to certain volcanic rocks of compact 

 and tufaceous structure, both as regards the nature and the arrange- 

 ment of the substances deposited. The siliceous products in the bricks 

 recall the (so-called) acid rocks, while the zeolites and palagonite 

 globules in the same way remind us of the (so-called) basic rocks. 

 These significant resemblances extend to the minutest particulars ; so 

 that even the most experienced mineralogist might mistake these 

 for natural products if the unaltered portions did not betray their 

 artificial origin. 



These relations of similarity and even of identity between the natural 

 and artificial products evidently justify us in assigning a common or 

 analogous mode of formation to both. Now, the conditions under which 

 the contemporary formation of zeolites accompanied by opal, chalce- 

 dony and carbonates proceeds are perfectly ascertained ; we have, so 

 to speak, presented to us in the products of these thermal springs the 

 results of a series of experiments of the most instructive character, 

 continued through a period of several centuries ; and that similar 

 agents have been at work in the production of similar minerals by 

 the alteration of erupted masses of rock, cannot reasonably be doubted. 

 In addition, however, to the class of rocks whose alteration forms 

 the subject of the present communication, I may remark that there 

 are others that have been transformed by similar agencies, but more 

 completely, such as palagonite, which, as is well known, is a hydrated 

 volcanic tuff in which concretionary masses of a peculiar kind replace 

 the crystals contained in the rock when in the unaltered condition. 

 Serpentine is probably another instance, the rock known by this name 

 being most likely the hydrated condition of masses which, in their 



* Metallurgy, vol. i. p. 318 (1875). 



