The Paragenetic Relations of Minerals, 87 



tified rocks cannot always have possessed the same state of 

 cohesion and density which they now present. Where the 

 geognostic characters of rocks put their sedimentary origin 

 beyond all question, it must not be supposed that their for- 

 mation consisted solely in mere mechanical deposition ; on 

 the contrary, it seems that in such cases chemical action has 

 not commenced until this has ceased. If, then, this is true 

 of the secondary and tertiary rocks, it is still more probable 

 with regard to those of more ancient date, which, there is 

 good reason to suppose, remained for long periods in a 

 softened condition. 



The conglomerates occurring in lodes, although not essen- 

 tially different, have been produced under somewhat modified 

 conditions. In this case, it is generally fragments of the ad- 

 joining rock which are imbedded in one or more crystallised 

 minerals. In the lodes near Freiberg, fragments of mica- 

 slate are found entirely imbedded in crystalline quartz, and 

 large masses of gneiss, perfectly detached from the adjoin- 

 ing rock, are found, especially near the roofs, covered with 

 the various minerals composing the lode, and arranged in 

 the same order as at the true saalbands. In the lodes of 

 Schlottwitz, near Dresden, which have suffered repeated dis- 

 locations, very sharp-edged fragments of band agate are 

 cemented together, and as it were imbedded in amethyst. 

 The lodes of red haematite in the upper Erzgebirge have suf- 

 fered similar dislocations, and fragments of this mineral are 

 now found imbedded in quartz. 



In all these instances, it appears obvious that the imbed- 

 ded substances are older than the matrices, and it is proba- 

 ble that the same view must be adopted with regard to that 

 class of agates which are surrounded by a crust of porphyry 

 sometimes of essentially different character to the mass in 

 which they are imbedded. These porphyry-agate balls were 

 perhaps originally adventitious fragments of another rock. 

 Hot aqueous vapour or other gases may have removed some 

 of their constituents, and left the silica in a hydrated opaline 

 state. There are some facts which greatly favour the opi- 

 nion that quartz has been formed by the contraction and 

 dehydration of opal. For example, in the lodes at Johann- 



