1818.} ON THE FORMATION OF ROCKS. 29i 



This is one of the most powerful and extensive for- 

 mations we know. The history of it alone would re- 

 quire a volume. It is found either in detached secon- 

 dary hills or ranges, (as the ridges that cross Eng- 

 land through Derbyshire) or lying on the flank of 

 primitive or transition mountains, as the immense cur- 

 tain whicli skirts the north and west side of the Car- 

 p^tthian, Bohemian, Tyrolean, and Alpine mountains;, 

 from the Black Sea to the borders of the Mediterrane- 

 an ; and the powerful and extensive beds which line 

 ilie basin of the Mississippi on the west side of the Al- 

 leghany mountains in North America. 



Where the stratum is very thick, tlie rock is solid 

 and compact, containing little or no shells or other or- 

 ganic matter ; but when the shells abound, the stratum 

 is tliinner, and the beds of shells, with some mixture 

 of argillaceous deposits, are found in greatest quantitiess 

 between the strata. These are often broken and irre- 

 gular, from the great number of excavations and ca- 

 verns they contain, through which run subterraneous 

 rivers, washing away the limestone, and deranging the 

 originally horizontal strata. 



There are great varieties in the colour ; the fracture 

 is sometimes earthy, but more frequently smooth, and 

 colichoidal. It appears to be mixed with a greater 

 quantity of depositions of impalpable argil or silex, 

 than the limestone with small grains of the transition 

 formation, which appears to be a purer crystalline 

 precipitate, and not so generally mixed with other 

 'earths, not chemically dissolved. 



The silex contained in this formation, is found geue- 

 nerally near the tops of the mountains in the upper 



