SPBINGS AND THOSE OBSERVED IN AMYGDALOIDS ETC. 81 



The deposits of agates, chalcedony, and other varieties of quartz in 

 volcanic rocks may he explained as the result of similar causes to 

 those producing these minerals by contemporary formation. These 

 rocks do not contain quartz or free silica as original components, but 

 have produced them as a consequence of their own alteration, in the 

 same manner as the opal and chalcedony formed in the concrete of 

 Plombieres, or as in the more striking case of the alteration of glass 

 when exposed to superheated water, whereby crystals of quartz and 

 diopside-augite are formed*. 



Siliceous infiltrations are found in many volcanic rocks of an 

 intermediate percentage of silica (rocks of the trachydoleritic class), 

 as for example in Auvergne, at Santorin, Aden, St. Paul and 

 Amsterdam Islands. In the so-called acid (trachytic) rocks these 

 infiltrations are occasionally so abundant that they become essentially 

 quartzose, passing into the condition of burr stones. Examples of 

 this kind of silicification are found in the trachytic tuffs of Tokay 

 and Hlinik, in Hungary, where the rocks, at times kaolinized, are 

 intermingled with quartz of different kinds, chert, amethyst, &c, 

 forming the so-called "porphyre molaire" of Beudant, and the hydro- 

 quartzite of Szabo, as well as various kinds of opal, including the 

 precious or noble opal which is worked for jewellery f . Similar facts 

 are observed in the island of Milo, where the millstone rocks contain- 

 ing opal and gelatinous silica are associated with cimolite and alunite. 

 Chlorite and green earth are other minerals occurring in this class of 

 rocks, which appear to be due to the same kind of action. 



Simultaneously with the production of green earth, quartz, and 

 other forms of free silica from the rock, by decomposition of its con- 

 stituent silicates the metallic bases of the latter, lime, magnesia, and 

 ferrous oxide, are reproduced as carbonates, forming the minerals 

 calcite, dolomite, and siderite, which, when present in such rocks, 

 have evidently been formed by the wet way. 



A very significant grouping, as regards the latter minerals, is ob- 

 served in the Roman masonry previously described, where we find 

 opal, sometimes in voluminous concretions, and less frequently chal- 

 cedonic quartz with lithomarge, calcite, aragonite, vivianite, and 

 fluor-spar. 



Zeolites are not exclusively confined to volcanic rocks, being also 

 found in certain metalliferous deposits, such as the silver-lead-ore 

 veins of Andreasberg, in the Harz, those of Kongsberg, in Norway, 

 and the reticulated groups of veins of the Banat. A very remarkable 

 instance of this class of association is afforded by the cupriferous 

 deposits of Lake Superior, where native copper is often disseminated 

 in prehnite, analcime, and other zeolites, as well as in calcite, in such 

 a manner as to show that the production of both kinds of minerals 

 has gone on under substantially similar conditions. 



The reason of the intimate association of compounds of such com- 



* Daubree, Mem. de l'Acad. des Sciences, 1860, vol. xvii. ; Ann. des Mines, 

 5e ser. vol. xvi. (1860) pp. 155, 193. 

 t Szabo, ' Trachyt &c. der Umgebung Tokay ; ' Naumann, ' Geognosie,' vol. 

 p. 618. 



Q. J. G. S. No. 133. * 



