﻿Xlviii PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



particularly apophyllite and chabasie*. Besides these, there are 

 some other zeolites, together with opal, niammillary hyalite (per- 

 fectly transparent and undistinguishable from the hyalite of basalts), 

 aragonite, and calcareous spar. The apophyllite is found only in 

 the cavities of the mortar; it is a silicate containing lime and potash ; 

 whereas the chabasie, a double silicate of alumina and potash, occurs 

 exclusively in cavities of the brick. The neighbouring granite has 

 never been found to contain any zeolites ; but the water contains the 

 elements of that family of minerals, potash and lime, the former 

 being derived from the decomposition of the felspar of the granite, 

 the latter from the mortar. Thus it appears that minerals which 

 we find in veins and in eruptive rocks may be formed by the joint 

 agency of water and heat, although that heat may not exceed 158° F. 

 In prosecuting these experiments, M. Daubree obtained well-formed 

 crystals of the transparent variety of augite, called diopside. Fir- 

 wood was changed into a black compact shining body, having all 

 the appearance of pure anthracite, and so hard as to be scratched 

 with difficulty by a steel point. It closely resembles the anthracite 

 that accompanies veins of silver in gneiss at Kongsberg in Norway. 



I have said that the experiments of Sir James Hall formed an 

 epoch in the history of theories of the earth. In the same light, but 

 in a still higher degree, the observations and synthetic experiments 

 of M. Daubree, showing the powerful and widely extended agency 

 of water, will probably be viewed as an epoch in our science. He 

 has demonstrated that water, accompanied by heat and compression, 

 with a very minute quantity of potash, has a solvent power upon a 

 wide range of mineral substances, especially upon silica, the earth 

 of all others the most prevalent in the structure of the globe. He 

 has further proved that the degree of heat imagined to be necessary 

 for the production of certain minerals has been much exaggerated, 

 and that products characteristic of metallic veins and volcanic rocks 

 may be formed at a temperature not exceeding 158° F. He refers 

 especially to the zeolite family so constantly found in trappean and 

 volcanic rocks, both as a constituent part, and as filling vesicles, 

 and which, therefore, may have been formed when the process of 

 cooling had been far advanced. He reminds us that zeolites have 

 been found in the fragments of tertiary limestone that occur in the 

 basaltic tuff of the Puy de la Piquette in Auvergne. He has further 

 shown that the molecular state of the water in lavas, be it what it 

 may, has had a great effect in the formation of silicates, even when 

 anhydrous. It causes them to separate, and to crystallize at a tem- 

 perature much below their point of fusion ; it enables them to cry- 

 stallize in an order of succession different from that of their fusibility: 

 thus, for example, leucite, an infusible silicate of ahunina and potash, 

 occurs in lavas in well-formed crystals, often of large size. To this, 

 Ludwig, in his German translation of Daubree's essay, adds that the 

 crystals of leucite often contain fragments of lava, and even small 



* In sixteen different species of zeolite, water has been found to enter largely 

 into the composition of each, varying from 8 per cent, in some specimens of 

 analcime, to nearly 22 per cent, in some specimens of chabasie. 



