490 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 5, 



tion of felspathic sediments, and are met with alike in the waters of 

 the unaltered Silurian schists of Canada and of the secondary 

 strata of the basins of London and Paris. In the purer limestones, 

 however, the felspathic or alkaliferous elements are wanting ; and 

 these strata often contain soluble salts of Hme or magnesia. These 

 would neutralize the alkaline salts which, infiltrating from adjacent 

 strata, would otherwise affect the transformation of the foreign mat- 

 ters present in the limestone into crystalline silicates. By a similar 

 process these calcareous or magnesian salts, penetrating the ad- 

 joining strata, would retard or prevent the alteration of these latter. 

 These considerations will serve to explain the anomalies presented 

 by the comparatively unaltered condition of some portions of the 

 strata in metamorphic regions *. 



§ II. As the history of the crystalline rocks becomes better known, 

 we find that many which were formerly regarded as exclusively 

 of plutonic origin are also represented among altered sedimentary 

 strata. Crystalline aggregates of quartz and felspar with mica offer 

 transitions from mica-schist through gneiss to stratified granites, 

 while the pyroxenic and hornblendic rocks of the altered Silurian 

 strata of Canada pass, by admixtures of anorthic felspars, into 

 stratified diorites and greenstones. In like manner the interstratified 

 serpentines of these regions are undoubtedly indigenous rocks, re- 

 sulting from the alteration of silico-magnesian sediments, although 

 the attitude of the serpentines in many countries has caused them 

 to be ranked with granites and traps, as intrusive rocks. Even the 

 crystalline limestones of the Laurentian series, holding graphite 

 and pyroxene, are occasionally found enveloping broken beds of 

 quartzite, or injected among the fissures in adjacent siliceous strata. 

 From similar facts, observers in other regions have been led to 

 assign a plutonic origin to certain crystalline limestones. We are 

 thus brought to the conclusion that metamorphic rocks, such as 

 granite, diorite, dolerite, serpentine, and limestone, may, under cer- 

 tain conditions, appear as intrusive. The pasty or semifluid state 

 which these rocks must have assumed at the time of their displace- 

 ment is illustrated by the observations of Daubree upon the swelling 

 up of glass and obsidian, and the development of crystals in their 

 mass, under the action of heated water, indicating a considerable 



* De Senarmont *, in his researches on the artificial formation of the mine- 

 rals of metalliferous veins by the moist way, has shown that, by aid of heated 

 solutions of alkaline bicarbonates and sulphurets, under pressure at 200" or 

 300° C, we may obtain in a crystalline form many native metals, sulphurets, and 

 sulph-arseniates, besides quarte, fluor-spar, and sulphate of barytes. 



Daubree 2 has since shown that a solution of a basic alkaline siKcate deposits 

 a large portion of its silica in the form of crystalline quartz when heated to 

 400° C. We have here, beyond a doubt, a key to the true theory of metal- 

 liferous veins. The heated alkaline solutions, which are at the same time the 

 agents of metamorphism, dissolve from the sediments the metalUc elements which 

 they contain disseminated, and subsequently deposit them with quartz and the 

 various spars in the fissures of the rock. 



1 Ann. de Chim. et de Phys. 3 s6rie, vol. xxxii. p. 129. 



2 Bull, de la Soc. Greol. de France, 2 serie, vol. xv. p. 99. 



