OEIGIN or CEYSTALLINB EOCKS. 29 



with regard to the latter, " it is evident that such silicates could be formed iu basins at the 

 earth's surface, by reactions between magnesiau solutions and dissolved silica;" a con- 

 sideration which was then applied to the generation of serpentine and of talc. Again, in 

 1863 and 1864, I ventured to conclude that " steatite, serpentine, pyroxene, hornblende, 

 and, in many cases, garnet, epidote, and other silicated minerals, are formed by a crys- 

 tallization or molecular rearrangement of silicates generated by chemical processes in 

 waters at the earth's surface." ^'■^ 



§ 56. "While natural waters hold in abundance both lime and magnesia, alumina is, 

 under ordinary conditions, insoluble in them, and moreover is not found uucombiued with 

 silica. The problem of the genesis of the aluminous double silicates, so abundant in the 

 rocks, was therefore a more difficult one than that of the simple protoxyd-silicates, with 

 which they are often intimately associated. Many facts in the history of zeolitic minerals, 

 however, soon led me to recognize in the conditions under which these aluminous double 

 silicates are formed, a clue to the solution of the problem. Thus it was that, in an essay 

 read before the Geological Society of Dublin, in April, 1863,"' I called attention to the 

 observations of Daubree on the production, during the historic period, of the zeolites, chabazite 

 and harmotome (phillipsite), by the action of thermal waters at a temperature not above 

 *rO° C, on the masonry of the ancient Roman baths at Plombières. The mode of the occur- 

 rence of these minerals showed that the aluminous silicate of the burned bricks had been 

 changed into a temporarily soluble compound, which had crystallized in cavities as 

 zeolites, which differ in composition from feldspars only by the presence of combined 

 water. I also called attention, in this connection, to the experiments of Daubree, who, by 

 operating at higher temperatures in sealed tubes, had succeeded in i^roducing crystallized 

 quartz, pyroxene, and apparently feldspathic and micaceous minerals. 



§ 5*7. The aqueous origin of feldspars, and their intimate relations to zeolites and 

 other hydrous minerals, were farther noticed by the author, in the "Geology of Canada," 

 in 1863, in which he cited the observations made by J. D. Whitney on the frequent 

 occuiTence of orthoclase in the copper-bearing veins in the melaphyres of Lake Superior. 

 The crystals of this mineral, which had been mistaken for stilbite, are there found under 

 conditions, which show their formation contemporaneously with the zeolites, analcime and 

 natrolite ; while elsewhere in the same region, the associates of the orthoclase are epidote, 

 calcite, native copper and quartz, upon which, as well as uj)on saponite, the crystals of the 

 feldspar were found implanted."'^ AVhitney recalled in this connection the occurrence of a 

 variety of orthoclase, the weissigite of Jenzsch, with chalcedony, in cavities of an amyg- 

 daloidal rock. 



§ 58. These facts were now insisted upon, in connection with my own observations, to 

 show the aqueous origin of the feldspar found in veins among the crystalline schists in the 

 province of Quebec, where " a flesh-red orthoclase occurs so intermingled with white 

 quartz and chlorite as to show the contemporaneous formation of the three species. The 

 orthoclase generally predominates, often reposing upon or surrounded by chlorite, and at 



^■- For citations and references see Chemical and Geological Essaj's, pp. 296, 297 and 300. 



5^ The Chemistry of Metamorphic Eocke ; Dublin Quarterly Journal for July, 1863 ; reprinted in Chemical and 

 Geological Essays, pp. 18-34. 



"Whitney, Amer. Jour. Science, 1869, xxviii., 16. 



