Geology, as illustrated by Chemistry and Physics. 51 



talline rocks which are supposed to have been formed by 

 plutonic metamorphism. But if, after the lapse of centuries, 

 there does not appear to be any tendency to a separation of 

 feldspar, then there is but little hope that they would be 

 formed even after thousands of years. * 



Another opportunity of judging the action of heat is afforded 

 by the fragments of sedimentary rocks imbedded in scoria- 

 ceous masses. In the Eifel, the neighbourhood of the lake of 

 Laach, at the Rodderberg, near Bonn, &c, such phenomena 

 occur very frequently ; for instance, fragments of slate and 

 graywacke imbedded in scorise. But here also, we recognise 

 nothing more than a similarity to burnt bricks. The lake 

 fragments frequently appear brick red, blistered at the edges, 

 sometimes like pumice-stone, and in some cases vitrified. 



The blistered character of such fragments shews that they 

 had been half melted ; from the large size of some of these, 

 we may infer a very gradual cooling ; Fakenberg, near Ber- 

 trich, 160 feet high, containing innumerable fragments of 

 slate-clay imbedded in it, may, according to an approximative 

 calculation, have required twenty-two years for its cooling. 

 More favourable circumstances for a plutonic metamorphosis 

 — a partial conversion of clay-slate into feldspar, the elements 

 of which it contains — could not be found, were such a change 

 possible, and still there has not even been a microscopic 

 crystal of feldspar found in these fragments."!" Every unpre- 

 judiced person, who is not blinded by attachment to an hypo- 

 thesis, will doubt the possibility of such a transformation, and 

 must do so until such a discovery shall be made. 



For these reasons, we are not justified in assuming such 

 a metamorphosis in sedimentary rocks, at depths where per- 

 haps the central heat of the earth may have produced a very- 

 high temperature, such as these scoriaceous masses possessed 

 in their former melted state ; since, as regards the action of 

 heat, it is indifferent whether they are of one or the other 

 origin. The thickest sedimentary formations, clay-slate 

 and graywacke, have undoubtedly a very considerable thick- 



* Lehrbuch der Chem. u. Physik. Geologie, Bd. IL, p. 354. 

 t Ibid., p. 733. 



d2 



