/'A'/.VC//'L/:S OF ROCK- WEATHERING 859 



position in silicate minerals, is greatly augmented when such 

 trituration takes place in connection with organic matter. J. Y. 

 Buchanan has shown" that the mud of sea bottoms is being con- 

 tinually passed and repassed through the alimentary canals of 

 marine animals, and that in so doing the mineral matter not 

 merel}- undergoes a slight amount of comminution and conse- 

 quent decomposition, but a chemical reduction takes place 

 whereby existing sulphates are converted into sulphides. Such 

 sulphides and the metallic constituents of the silicates and other 

 compounds, particularly those of iron and manganese, would on 

 exposure to sea water become converted into oxides. It is 

 through such agencies that he would account for the presence of 

 sulphur in marine muds, and the variations in color, from shades 

 of red or brown to blue and gray, in the former the iron occur- 

 ring as oxides, while in the latter it exists as a sulphide. Of 

 course either form may be more or less permanent according as 

 the mud ma)- be devoid of animal life, or protected from oxidiz- 

 ing influences. 



3. ANALYSES OF FRESH AND DECOMPOSED ROCKS. 



Let us now take into consideration a few common rock types 

 which have undergone a process of degeneration through 

 weathering, and by means of analyses ascertain, so far as pos- 

 sible, the chemical and physical changes which have taken 

 place. In the table below are given, in each case, the results of 

 analyses of fresh and decomposed materials, and the calculated 

 percentage loss of constituents, both as relates to the entire rock, 

 and to the individual constituents. In making these calculations 

 it has been necessary to assume that one of the constituents 

 remains practically constant, in order that it may serve as a 

 basis of comparison. That constituent which has been shown 

 by a large series of analyses to be most constant is, among 

 siliceous crystalline rocks, the alumina, though sometimes it is 

 the iron. Among calcareous rocks it is the silica. In an}^ case, 



M)n the Occurrence of Sulphur in Marine Muds. Proc. Royal Soc. cf Edinburgh, 

 1S90-1. 



