!£!«.] ON THE FOUMATION OF ROCKS, 295 



tached fragments require some observation^ to unite and 

 reduce to one general formation ; with this necessaiy 

 attention, it will be found to be rather an extensive for. 

 mation, as it is in North America, covering indiscrim- 

 inately different kinds of the primitive, from Coniie :ti- 

 cut River to the Rappahanock, nearly 150 leagues. 

 On both sides of the Yosges to beyond Treves, it gen- 

 erally reposes on the porphyry, covering the porphyry 

 of the chain of mountains in the black-forest opposite 

 the chain of the Vosges, and equally covering the por- 

 phyry on the south side of Tyrol, from the valley of 

 Falsa, to near Bergamo, and perhaps farther, as the 

 same porphyry lies upon the gneiss on Lago Majore, 

 though there the sandstone is Avanting, perhaps from 

 being washed away. 



As this formation has been taken for the graywacke, 

 and graywacke sliist, by some mineralogists, it may not 

 be useless to give here a description of the points in 

 which they resemble, and the properties wherein they 

 differ, according as I have observed them. 



These two formations resemble each other in being 

 united by a cement consisting mostly of argil, and tak- 

 ing the appearance of clay slate, when the cement a- 

 bounds either in the sand stone or puddings. TMb 

 sandstone likewise, as in the graywacke shist, takes 

 a shistoze appearance, with particles of clay slate, when 

 the cement predominates; and in situation, it is immedi- 

 ately following the primitive, like the gray^ acke shist 



and other transition rocks. 



• 



The two formations differ in colour; the red sand- 

 stone cement containing a considerable quantity of the 

 j-ed oxide of iron; in hardness, the red sandstone being 



