78 PROF. A. DAUBBEE on incrustations by thermal 



to find these minerals completely developed in form in the wet 

 way at a temperature which is far below the point at which they 

 are reputed to be soluble in water. 



The nature of the minerals produced is very closely connected with 

 variations in the composition of the containing material ; thus the 

 crystals of chabasite (a non- calcareous zeolite) are almost entirely 

 confined to the bricks, while others are more abundant in the mortar, 

 the same solution being capable of producing different minerals by 

 its action upon rocks of different characters. This localization of 

 typical zeolites seems therefore to prove that the constituents were not 

 entirely introduced by the mineral-water, but that the lime, alumina, 

 and other substances necessary to the constitution of the new com- 

 pounds were furnished partly by the bricks and partly by the mortar, 

 the chief constituents furnished by the mineral-water being the 

 alkalies. The newly developed minerals are generally in close 

 proximity to their points of formation, in which respect they differ 

 from the greater part of the minerals found in metalliferous veins, 

 which are often completely absent from the rocks forming the walls. 

 It may be supposed, however, that water acting similarly at a higher 

 temperature would exert a greater solvent power, and therefore 

 that zeolites produced under such conditions might be susceptible of 

 removal to a greater distance from their points of origin than in the 

 case under discussion. 



Importance of Zeolites and the associated Minerals as Secondary Products 

 in Volcanic Rocks, especially in Amygdaloids and Conglomerates. 



The fact that silicates of the zeolitic group are found under exactly 

 similar conditions in the so-called amygdaloidal rocks in countries 

 far removed from each other is well known to all mineralogists ; and 

 the same resemblance extends to rocks of different geological ages, 

 such as the melaphyres and diabases of the Silurian, Permian, and 

 Triassic periods, and the basalts of Tertiary times. The characters of 

 these minerals lead to the conclusion that they have been developed in 

 the vesicles and other cavities of the rock subsequently to its consolida- 

 tion. The most widely distributed species of the group, stilbite, 

 heulandite, chabasite, mesotype, analcime, and christianite, are found 

 not only associated together in the same mass of rock, but often in 

 the same cavity, following a recognizable order of succession. 



It is usual to find other minerals in association with zeolites, which 

 appear to have been developed under conditions but little different 

 from those that have produced the latter. The most important of 

 these associated minerals is quartz, in the various forms of agate, 

 chalcedony, and crystallized quartz — others being opal, different kinds 

 of chlorite, delessite, chalcedonite, and chloropheeite, green earth, 

 lithomarge, steatite, clatholite, and epidote, together with calcite, 

 aragonite, and siderite ; and, lastly, among their rarer associates 

 are vrvianite, barytes, celestine, fluor-spar, native copper, &c. 



Among the localities producing zeolites and their associated 

 minerals the following may be mentioned : — 



