ft M. Mohs's Summary of Geogmstical Phenomena, 



however, disappears entirely in gneiss itself, while the others 

 are distinctly seen.* 



It is now easy to make the application to the tabular masses 

 into which granite appears to be sometimes divided. The sur- 

 faces which bound these, have precisely the characters of the 

 surfaces of distinct concretions, and the divisions considered as 

 strata, are here nothing else but tabular distinct concretions. 

 The appearance can be explained without difficulty. We fre- 

 quently find, particularly in masses of limestone, that a two- 

 fold composition ( zusammensetzung ) or structure is present, 

 from which tabular distinct concretions can be produced, and 

 that sometimes the one, sometimes the other, is more deve- 

 loped and becomes more apparent, or that it becomes more 

 and more indistinct, and disappears ; whence proceed many, at 

 first sight, apparently remarkable phenomena. This may also 

 be supposed in granite, in order to understand easily in what 

 connection the tabular distinct concretions stand with the 

 massive, and in order to be convinced that the one proceeds 

 from actual stratification just as little as the other. 



The granite masses, in whatever form they appear, are ge- 

 nerally surrounded by thick masses of slaty rocks. The cases 

 in which other features present themselves, may here be 

 passed over. The slaty rocks exhibit the extremely remark- 

 able phenomenon, that, in respect to their structure, and the 

 relations of their beds on the great scale, they present the same 

 features as if granite-masses were not jjresent. It was believed 

 that the slate rocks surround the granite masses in a mantle - 

 shaped manner, and that the latter had exercised such an in- 

 fluence on the first, as to determine the form of arrangement 

 of their beds. But further investigations have proved that 

 this occurrence so rarely takes place, that, where it has actu- 

 ally been found by accurate observation to occur, it may be 

 merely regarded as an exception ; and that, on the contrary, 



* Thus gneiss, and most of the slaty and other mountain masses are not 

 stratified, but the tabular divisions observable in them are referable to struc- 

 ture, and are mere distinct concretions. TJie criteria for distinguishing real 

 from apparent stratification, derivable from the ideas of the formation of this 

 rock, cannot be fully discussed in this place. "What has been already said 

 is sufficient to shew, how necessary it is to take care not to regard all as 

 etratification which possesses its external aspect. 



