494 PEOCEEDIlSrGS OF THE GEOLOGICAL SOCIETY. [Jan. 5y 



magnesia from sediments, and leave the granitic elements inter- 

 mingled with oxide of iron ; while, on the other hand, hy the 

 admixture of organic materials, the whole of the iron may he re- 

 moved from strata which will still retain the lime and soda necessary 

 for the formation of basic felspars. The fact that bicarbonate of 

 magnesia is much more soluble than bicarbonate of lime, is also to 

 be taken into account in considering these reactions. 



The study of the chemistry of mineral waters, in connexion with 

 that of sedimentary rocks, shows us that the result of processes 

 continually going on in nature is to divide the silico- argillaceous 

 rocks into two great classes, — the one characterized by an excess of 

 silica, by the predominance of potash, and by the small amounts of 

 lime, magnesia, and soda, and represented by the gTanites and 

 trachytes, while in the other class sihca and potash are less abun- 

 dant, and soda, lime, and magnesia prevail, giving rise to triclinic 

 felspars snd pyroxenes. The metamorphism and displacement of 

 sediments may thus enable us to explain the origin of the different 

 varieties of plutonic rocks without calling to our aid the ejections of 

 the central fire. 



§ VI. The most ancient sediments, like those of modern times, 

 were doubtlessly composed of sands, clays, and limestones, although, 

 from the principles already defined in § lY. and § Y., it is evident 

 that the chemical composition of these sediments in diff'erent geologic 

 periods must have been gradually changing. It is from a too hasty 

 generalization that an eminent geologist has concluded that lime- 

 stones were rare in earlier times ; for in Canada the Laurentian 

 system — an immense series of stratified crystalline rocks which 

 underlie unconformably both the Silurian and the old Cambrian or 

 Huronian systems — contains a limestone-formation (interstratified 

 with dolomites), the thickness of which Sir "VY. E. Logan has esti- 

 mated at not less than 1000 feet. Associated with this, besides 

 great volumes of quartzite and gneiss, there is a formation, of vast 

 but unknown thickness, the predominant element of which is a 

 triclinic felspar, varying in composition between anorthite and 

 andesine, and containing lime and much soda, with but a small pro- 

 portion of potash. These felspars are often mixed with hypersthene 

 or pyroxene; but great masses of the rock are sometimes nearly 

 pm-e felspar. These felspathic rocks, as well as the limestones, are 

 associated with beds of hematitic and magnetic iron-ores, the latter 

 often mixed with graphite. Ancient as are these Laurentian rocks, 

 we have no reason to suppose that they mark the commencement of 

 sedimentary deposits : they were doubtlessly derived from the ruins 

 of other rocks in which the proportion of soda was still gTeater ; and 

 the detritus of these Laiu-entian felspars, making up our palaeozoic 

 strata, is now the source of alkaline waters by which the soda of the 

 silicates, rendered soluble, is carried down to the sea in the form of 

 carbonate to be transformed into chloride of sodium. The lime of 

 the felspais being at the same time removed as carbonate, these sedi- 

 mentary strata in the course of ages become less basic, poorer in 

 soda and lime, and comparatively richer in alumina, silica, and 



