106 The Paragenetic Relations of Minerals. 



metals, while the heavier metals, existing perhaps chiefly as 

 sulphurets, sunk towards the centre. This opinion gains some 

 amount of probability, from the high specific gravity of the 

 earth (according to Reich, 5*45 ; Baily, 5'66), while the mean 

 specific gravity of all rocks is only 2- 75. When, at subse- 

 quent periods, fissures were formed in the superficial parts 

 of the earth, they might have been filled by eruptive sulphu- 

 rets, &c. Most of the metalliferous minerals are or have 

 been in the state of sulphurets ; and the old belief of miners, 

 that in general lodes are richer the deeper they are worked, 

 which has now gained considerable probability upon scienti- 

 fic grounds, likewise favours the above view. 



There can be no doubt that sometimes, although rarely, 

 the substances present in lodes have at least partly been in- 

 troduced from above. Instances are known in which some 

 of the chemical constituents of the minerals must have come 

 from the surface. For example, the phosphoric acid in pyro- 

 morphite, wavellite, kaelaite, &c. There are, however, very 

 serious objections to Werner's theory, that all the substances 

 present in lodes were introduced from the surface. 



The study of mineral lodes has led to the assumption of 

 four different modes of formation : — 



(A.) Congeneration. 



(B.) Lateral Secretion. 



(C.) Ascension ; and 



(D.) Descension. 

 The two latter have, however, been the most frequent. It 

 is also probable that, in some instances, two or more of these 

 modes of action have gone on together. 



Many lodes have, since their formation, suffered alteration 

 to such an extent that the substances they contain are all 

 products of the decomposition of the original minerals ; and 

 sometimes these products demand a special attention. 



B. H. Paul. 



(To be continued.) 



