^92 ON THE FOILMATION OF ROCKS. f July, 



strata^ either in detached rounded masses, or in thin 

 strata, and follows the colour of the limestone. When 

 red, the silcx is also red, as on Monte Baldy*, where the 

 limestone is blue, the silex is generally of a dark black- 

 ish colour, it is mixed and rather of a lighter con 

 lour v/hen the limestone approaches to white. 



17. Sandstone in beds, independent of other for- 

 mations, contains often the impressions of shells. The 

 calcareous matter of tlie shell is generally washed away, 

 and perhaps helps to form the cement. Being an aggre- 

 gate of particles of rocks rounded by attrition, it must 

 be considered as of Neptunian origin, though in some 

 Instances the sand may be heaped together by th^ wind 

 ?is well as by the water. 



This formation is not very extensive, covering the foot 

 of the compact calcareous hills, or pai-tially accorapa- 

 nying vales or lower situations; it often hardens by ex- 

 posure to the air, and remains divided into high and 

 massive pillars, as on the south side of the mountains 

 in Bohemia near Abernach; or in long dikes, as what 

 is called the Devil's dike, at the foot of the Hartz; it 

 constitutes considerable ridges in the basin of the Mis- 

 sissippi; and in lower situations, where it covers thelimcr 

 stone, it is impregnated with from 10 to SO per cent of 

 native nitre, and large masses of pure nitre have been 

 found included in the sandstone in the state of Ken- 

 tucky, North Aijierica^ 



48. Pitddingsto7ie formed of pebbles rounded by 

 attrition, and including (though not frequently) the 

 ^emai^s of organic matter, must be considered of Nen-: 



