ON INCRUSTATIONS BY THERMAL SPRINGS ETC. 73 



7. On Points of Similarity between Zeolitic and Siliceous Incrusta- 

 tions of recent Formation by Thermal Springs, and those observed 

 in Amygdaloids and other altered Volcanic Rocks. By Prof. 

 A. Daubree, E.M.G.S. (Eead June 20, 1877.) 



[Plate IV.] 



The production of zeolitic minerals by the agency of thermal waters 

 is not, as might at first sight have been supposed, a merely accidental 

 circumstance confined to a particular locality, but appears to be 

 tolerably general in many places — and is therefore of particular in- 

 terest, not merely as illustrating the changes that have gone on at 

 the springs themselves, but as furnishing a clue to the origin of 

 many of the components of erupted rocks, and more especially of 

 altered volcanic rocks. 



I. Zeolites and allied minerals produced by Thermal Springs. 



The locality where these minerals were first observed as deposits 

 from hot springs is Plombieres* (Vosges); and subsequently I 

 have discovered them under similar conditions in Roman masonry 

 at Luxeuil f (Haute- Saone), at Bourbonne (Haute-Marne), and in 

 the neighbourhood of Oran in Algeria. At all these places the 

 mouths of the thermal springs, when first utilized for baths during 

 the Roman occupation, were surrounded by works in concrete formed 

 of fragments of brick and stone, both sandstone and limestone, united 

 by lime mortar. When, in the progress of recent works, it became 

 necessary to cut into these masses of ancient masonry, it has been 

 found that certain portions of them have, under the action of the 

 mineral- water, undergone changes of a very remarkable kind, both 

 in a chemical and mineralogical point of view, and of the following 

 character X- 



In the vesicular cavities of the bricks new minerals are promi- 

 nently developed, being principally zeolites, among the most abun- 

 dant of which is chabasite in limpid crystals having the form of a 

 rhombohedron approximating to a cube, which are striated parallel to 

 the edges, and at times appear in the macles common to the species. 

 The measurement of the angles, as well as chemical analysis, show 

 no differences from the natural mineral. Phillipsite or lime harmo- 

 tome is similarly found accompanying the preceding species — an 

 association exactly analogous to that observed in the amygdaloidal 

 trap rocks of the west of Iceland by M. Descloizeaux §. 



In the hollows of the calcareous cement, small crystals of apo- 

 phyllite are .found which correspond to the natural mineral in form 

 and composition, even to giving traces of fluorine when heated in a 



* Coniptes Kendus, xlvi. pp. 1086, 1201 (1858). 



t Ann. des Mines, 5th series, vol. xiii. p. 227. 



\ Bull, de la Soc. Geol., 2nd series, vol. xvi. p. 502, xii. p. 562, xviii. p. 108. 



§ Ann. des Mines, 4th series, vol. xii. p. 373. 



