88 The Paragenetic Relations of Minerals. 



georgenstadt (Saxony), small masses of white opal occur im- 

 bedded in hornstone, close to somewhat larger quartz druses 

 in the same hornstone. These quartz druses are perfectly 

 closed, and sometimes covered with an exterior crust of opal, 

 and, when broken, are found to contain water. It is there- 

 fore highly probable that these agates, which differ widely 

 from those formed by infiltration, have been formed by the 

 contraction of opal. 



The occurrence in eruptive rocks of imbedded masses, 

 undoubtedly adventitious, and whose original condition 

 may in some instances be recognised, is very frequent. 

 The so-called basalt jasper consists of fragments of some 

 foreign rock. The sandstone of the " Blauen Kuppe" (Hesse 

 Cassel) is more or less altered at its contact with basalt 

 approximating in character to basalt jasper. The basalt jas- 

 per of Johanngeorgenstadt, and that near Eisenach, are more 

 homogeneous, but streaks resembling those of the gneiss from 

 which they have originated are still perceptible. At Hohen 

 Borkenstein (Bavaria), the fragments of basalt jasper con- 

 tain crystals of felsite which communicate to it a porphyritic 

 appearance. It is, however, very probable that the altera- 

 tion of rocks, consisting chiefly of silica and alumina, into 

 basalt jasper, has not been caused by volcanic heat alone, 

 but perhaps more essentially by the introduction of sub- 

 stances from the basalt. 



These facts will sufficiently shew, that with regard to the 

 phenomena of adventitious admixtures no definite mode of 

 association can be recognised, it being entirely a matter of 

 chance that an eruptive rock tears away and encloses frag- 

 ments of those through which it penetrates, nor have these 

 phenomena any further connection with the present subject, 

 than as serving to prove that the substances imbedded in 

 mixed rocks or simple minerals may be of very different 

 origin, and that any exterior resemblance, especially of 

 form, is insufficient by itself to justify the inference of a 

 similar mode of formation. Thus, the nodules of olivine and 

 basalt jasper occurring in basalt, and alike both in shape and 

 size, have certainly originated by very different processes. 



In some rare instances the matrix of a porphyritic rock 

 would appear to be the most recent. Both Darwin andCredner 



