On the Paragenetic Relations of Minerals. 59 
with regard to their formation the question arises, whence 
did their molybdic acid originate? It is scarcely admissible 
to assume that molybdic acid or molybdate of lead were 
introduced into the lodes ready formed, since under all cir- 
cumstances the molybdates are implanted upon the galena, or 
occur as pseudomorphs after it, like other minerals of the 
kind, with regard to the source of whose acids there is no 
doubt. It would seem that the molybdenum previously 
existed in some decomposed mineral which has not as yet 
been met with. Chemical examination of the galena itself 
did not show the presence of any molybdenum in it. 
The zine-blende of this formation is generally of the 
brown, rarely of the red or yellow colour, and never black. 
_ Copper pyrites occurs in small quantity as one of the oldest 
members of this formation. A great variety of decomposition 
products of it and of galena conjointly occur, particularly in 
the upper parts of the lodes.- 
XV. Barytic Copper Formation.—It has already been 
pointed out that copper pyrites occurs frequently in forma- 
tions whose principal bedding is barytite. Variegated and 
ordinary copper pyrites, together with copper-glance, occur 
in considerable masses in barytite lodes, without any other 
minerals so.as to constitute what may justly be regarded as 
a special formation. 
XVI. Silver Formation.—As in the previously mentioned 
quartz formation, the argentiferous and auriferous pyrites 
are almost invariably associated with mispickel and quartz, 
they may be regarded as constituting a separate formation, 
more especially as their age is undoubtedly very great. 
However, argentiferous minerals are elsewhere more abun- 
dant, and so decidedly associated with and accompanied by 
the very recent barytite, that they may likewise be regarded 
as a separate formation. It resembles the pyritic formation 
in being frequently transferred into the adjoining rock, 
although not in such considerable masses, and the rock is 
less disintegrated. Silver glance has been found between 
the planes of cleavage of undecomposed gneiss. The minerals 
containing gold or silver which belong to this formation are 
generally of very recent formation, and are consequently 
