M. Keilhau s Theory of' Granite and other Rocks. 2G5 



that either do not occur at all, or not in sufficient quantity for 

 the newly formed minerals, in those masses which have under- 

 gone the metamorphoses ; whereas the latter consist partly of 

 substances of which at least not much (carbonate of lime has 

 been noticed in granite) is discovered in the changed rock. In 

 what manner this exchange is to be conceived, it is for the pre- 

 sent impossible to explain more precisely ; but I further regard 

 it as much more natural to suppose that no new material was 

 transported from without to produce the converting action to 

 which such regions were subjected, than to assume that there 

 were really cases where it must be imagined that sublimations 

 of potash, silica, &c. actually came from the deeply-seated vol- 

 canic laboratory. 



The great porphyry formations occur principally in the sand- 

 stone, which forms the deposit above the clay-slate and lime- 

 stone. It is probable that the action took place especially on 

 the same fine grained partly earthy sandstone-formation, often 

 almost passing into iron-clay (Jernleer), of which there is still 

 much remaining in an unaltered state, both in the porphyries 

 and in other situations. When the sandstone had the character 

 of conglomerates, there thus arose, at many points at least, a re- 

 sistance to the transforming action : in this manner we often find 

 certain layers filled with small quartz pebbles, directly under 

 those porphyry masses, whose development in the uppermost 

 sandstone etage was interrupted by these coarse strata. Beauti- 

 ful proofs of the tranquil formation of the porphyry arc afforded 

 by the strata as well of the clay-like sandstones^ as of the con- 

 glomerates, for these stretch far into the porphyries, having 

 the same gentle inclination which is so general in the sand- 

 stone strata. Particular strata also, which are now fairly in- 

 cluded in the massive mountain-rock, stand partly in connec- 

 tion with that sandstone basis, from whose range of strata we 

 can follow a long portion projecting like a plank, and can at the 

 same time convince ourselves that it is not in the slightest de- 

 gree broken or bent ; but even when the connection with the 

 body of the strata is broken off by the passage into the por- 

 phyry which also takes place, we can in like manner ascertain 

 distinctly, by the angle of inclination, the unaltered position of 

 the portions of strata, still in the form of sandstone, lying in the 



