OEIGIN OF CEYSTALLINB EOCKS. 57 



nature is effected slowly at much lower temperatures, aud that the formatiou of the 

 hydrous double carbonate already described is, perhax)s, au intermediate stage in the 

 process.'"" 



§ 114. The reactions described in the preceding paragraphs between the elements of 

 comparatively insoluble substances in the presence of water, resulting not only in the con- 

 version of amorphous into crystalline bodies, but in the breaking-up of old combinations, 

 as well as in the union of unlike matters mechanically mingled to form new crystalline 

 species, are iustnictive examples of what Griimbel has termed diagenesis. The changes 

 in the masonry of the old Roman baths in contact with thermal waters, resulting in the 

 hydration of the substance of the bricks, and its conversion into zeolitic minerals ; the 

 hydration of volcanic glasses, with similar results, going on, even at low temperatures, 

 in the deep sea ; the decomposition of common glass by heated water ; the conversion 

 of basaltic rock into palagonite and the production therefrom of zeolites ; the similar 

 changes seen elsewhere in amygdaloids, aud even in massive basic plutonic rocks, are also 

 examj^les of this process of diagenesis, and serve to show its great geological significance. 

 We have already suggested the intervention of similar reactions in past ages among the 

 sediments from the subaërial decay of felspathic rocks, in some cases with the concurrence 

 of the secretions from the primary basic stratum, which, in accordance with the crenitic 

 hypothesis, we suppose to have been the source of soluble mineral silicates. In the 

 diagenesis of these early argillaceous sediments, aided by crenitic action, will, it is 

 believed, be found the origin of many of the crystalline schists of the transition rocks. 



§ 115. An instructive phase in this diagenetic process is that of the gradual conversion 

 of smaller crystalline grains or crystals into larger ones, which is familiar to chemists. 

 This action is in fact nearly akin to that which takes place in the transformation of amor- 

 phous into crystalline precipitates, since in both cases a partial solution precedes the 

 crystallization. It is well known that, as a result of successive solution aud re-deposition, 

 large crystals may be built up at the expense of smaller ones. This process, as H. Deville 

 has shown, " suffices, under the influence of the changing temperature of the seasons, to 

 convert many fine precipitates into crystalline aggregates, by the aid of liquids of slight 

 solvent powers. A similar agency may be supposed to have effected the crystallization of 

 buried sediments, and changes in the solvent power of the permeating water might be 

 due either to variations of temperature or of pressure. Simultaneously with this process, 

 one of chemical union of heterogeneous elements may go on, and in this way, for example, 

 we may suppose that the carbonates of lime and magnesia become united to form dolomite 

 or magnesian limestone." "" 



§ 116. The tendency ol the dissolved material in this process to crystallize around nuclei 

 of its own kind, rather than on foreign particles, is a familiar fact, and its geological import- 

 ance, to which I first called attention, as above, in 1869, was again pointed out by Sorby in 

 1880, when he showed that dissolved quartz might be deposited upon clastic grains of 

 this mineral in perfect optical and crystallographic continuity, so that each broken frag- 

 ment of quartz is changed into a definite crystal, as was seen in his microscopic 



™ Hunt, Contributions to the Chemistry of Lime and Magnesia, part i., 1859. Amer. Jour. Sci., xxviii., pp. 

 170,365, and part ii., 1866, vol. xii., p. 49 ; also in abstract in Chem. and Geol. Essays, pp. 80-92. 



"» Hunt, The Chemistry of the Earth, Report Smithsonian Institution, 1869 ; also Chem. and Geol. Essays, p. 305. 



Sec. III., 1884. 8, 



