^66 M. Keilhairs Tlieory of Granite and other Rocks. 



porphyry. This fact was not touched on formerly by us ; it 

 likewise belongs to those which volcanists cannot perceive. 



Porphyry, with its more or less uncrystalline basis, does not 

 appear to have been so much adapted as the granites and syenites 

 to form thin branches proceeding in every direction into the 

 bounding strata : continuations of the principal mass of the 

 porphyry are certainly met with as veins situated in the sub- 

 jacent strata, but these veins are so large, and of such solid 

 forms, that they cannot be at all compared to the multitudes 

 which ramify from the granitic masses. 



The more subordinate crystalline siliceous rocks occurring 

 in the stratified transition series, were probably developed at 

 the same period, and in the same manner, as those larger masses. 

 That the transforming action operated especially on certain 

 beds, and that the nature, and perhaps also the position, of these 

 beds had a particular influence on the metamorphic rock which 

 made its appearance, seems extremely natural ; and I now al- 

 lude particularly to eurite porphyry, which so often occurs in 

 the form of beds. But as to deviations from occurrence in the 

 form of beds, there is nothing very peculiar, when we take into 

 consideration dolomites, granites, &c., and are accustomed to 

 observe the readiness with which it appears that the processes 

 have spread around, and with which they have taken particular 

 directions during their progress through the masses encountered ; 

 — a readiness which, however, seems limited in other cases, in- 

 asmuch as metamorphoses at particular points have evidently 

 been checked by insuperable difficulties ; and certain formations 

 appear to have had a greater tendency to assume certain exter- 

 nal forms, suited to their internal constitution. 



The crystalline granular siliceous rocks occurring as inde- 

 pendent veins, that is, as veins which do not branch out from the 

 large masses, viz. greenstone veins with their parallel sides, are 

 certainly greatly opposed to our theory ; and indeed that theory 

 will seem to many at once to deserve rejection, when they re- 

 member the remarkable fact of gneiss fragments occurring in 

 the above-mentioned veins. I grant that the difficulties are 

 considerable ; but even should this single case be quite insur- 

 mountable, yet still I do not find, on that account, that there 

 is any sufficient ground for giving up the proposed views, and 



