IS M. Mohs's Summary of Geog not Heal Phenomena, 



dual component parts can no longer be distinguished from one 

 another. The transitions to be observed prove this. As the 

 granites, in reference to the relative quantities of their con- 

 stituent parts, are so extremely different, porphyries with dif- 

 ferent kinds of basis are thence produced, which are distin- 

 guished as hornstone, felspar, and claystone porphyries. Oc- 

 casionally, portions of granite, syenite, or gneiss, arise in the 

 general porphyry-mass, partly as irregular masses, partly as 

 apparent fragments, thus affording a direct proof of the ori- 

 gin of the porphyry. Porphyries appear as beds, as veins, 

 and also as irregular masses in slate-rocks ; and the last are 

 often of such extent, that not only do slate and other moun- 

 tain masses arise from among these porphyries, also in more or 

 less regular bed-like forms, but they supplant the latter for con- 

 siderable distances. The beds and veins of these mountain-mas- 

 ses do not appear to be of much importance in reference to the 

 discovery of useful minerals ; but their large masses, on the con- 

 trary, are worthy of particular attention, because, as we know 

 from experience, in many districts they are very rich in ores. 

 At their meeting with the slate series, they present the same 

 phenomena as granite does, inasmuch as both the rocks in con- 

 tact are partly reciprocally altered, partly pass directly into each 

 other, just likewise as similar passages take place between gra- 

 nite and porphyry, and these sometimes are so gradual, that we 

 must pass through a great number of varieties before we find 

 the transition completed. Very frequently, extensive masses 

 of porphyry replace the whole, or, at least, a portion of the 

 slate rocks. The consequence of this is, that generally on one 

 declivity of a mountain-group there are no porph)T['ies what- 

 ever, and we meet only with slaty rocks ; whereas on the 

 other side, large tracts of porphyry present themselves. The 

 porphyries also pass into sandstones, viz. the red sandstones, 

 and into conglomerates, whose cement is often very much like 

 sandstone, and which alternate many times in regular beds with 

 the most characteristic porphyries. Phenomena of this de- 

 scription may undoubtedly create doubt as to the generally 

 received opinion of the nature of such conglomerates, I mean 

 the opinion that they consist of pebbles derived from previ- 

 ously existing mountain-masses ; but such discussions are not 



