On Some Points in Chemical Geology, 421 



Waters deficient in organic matters may remove soda, lime and 

 magnesia from sediments, and leave the granitic elements inter- 

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

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

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

 for the formation of basic feldspars. 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 granites and 

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

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

 and triclinic feldspars. 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 IV. and V., it is evident 

 that the chemical composition of these sediments in different 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 W. 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 feldspar, varying in composition between anorthite and 

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

 portion of potash. These feldspars are often mixed with hypersthene 

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

 pure feldspar. These feldspathic 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 



