416 On Some Points in Chemical Geology. 



the unaltered Silurian schists of Canada, and of the secondary 

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

 however, the feldspathic or alkaliferous elements are wanting ; and 

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

 would neutralize the alkaline salts, which infiltrating from adjacent 

 strata, might otherwise effect the transformation of the foreign mat- 

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

 process these calcareous or magnesian silts, penetrating the ad- 

 joining strata, would retard or prevent the alteration of the 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 feldspar 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 feldspars, 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 



|| De Senarmont 1 in his researches on the artificial formation of the min- 

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

 heated solutions of alkaline bicarbonates and sulphurets, under pressure 

 at temperatures of 200° or 300° C, we may obtain in a crystalline form 

 many native metals, sulphurets, and sulpharseniates, besides quartz, 

 fluor-spar and sulphate of barytes. 



Daubrde 2 has since shown that a solution of a basic alkaline silicate 

 deposits a large portion of its silica in the form of crystalline quartz 

 when heated to 400° C. We hare here, beyond a doubt, a key to the 

 true theory of metalliferous veins. The heated alkaline solutions, 

 which are at the same time the agents of metamorphism, dissolve from 

 the sediments the metallic elements which these 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), vol. xxxii, p. 129, 

 2 Bull. Soc. G£ol. de France (2), vol. xv, p. 99. 



