FliCETZ TRAP FORMATIONS, 197 



nised beings. It is also less distincth stratified than the va- 

 riety before mentioned, and contains many beds ot purjdins;- 

 stone (le |)Oudingue of Brochant.) Tliese beds are some- 

 times of great extent, but certainly cannot be considered as 

 constitiitin^j; a stratum distinct from the sandrock. Tiiis 

 tract of inclined sandstone, which skirts the eastern boundary 

 of the Rocky Mountains, and which we have considered as 

 constituting a part of that immense formation of secondary 

 which occupies the valley of the Mississippi, abounds in sce- 

 n<'ryof a grand and interesting character. The great incli- 

 nation of the strata we have before noticed. That side of 

 the ridges which is nearest the primitive, appears as if bro- 

 ken off from a part of the stratum l)eyond, and is usually an 

 altrupt perpendicular precipice, sometimes overhanging and 

 sheltering a considerable extent of surface. 'I'lie uppei- part 

 of the stratum, or that surface which is most distant from the 

 primitive, is usually smooth and hard, and both sides arc 

 alike destitute of soil and verdure. Elevations of tiiis sort 

 are met with varying from twenty to several thousand feet 

 in tliickness. Nor are they by any means uniform in height. 

 Some ol'then\ rise probably tliree or four hundred feet, and 

 considering their singular character, would be high, were 

 they not subjected to an immediate and disadvantageous 

 comparison with the towering Andes at whose feet they are 

 placed. Their summits, which in some instances are regu- 

 lar and hoiizontal. are crowned with a scanty growth of ce- 

 dars and pines. Where the cement, and most oltiie ma- 

 terials which constitute the sandstone, arc silicious. the rock 

 evinces a tendency to separate in fragments of a rhombic 

 form : and in this instance the elevated edge often presents 

 a notched or serrated surface. Those sandstones which con- 

 sist of silex with the least intermixture of foreign ingredients 

 are the most duial)le. jRut in the region which we are now 

 consi.lerinir, tlie variations in tlw composition, character, and 

 cement of the sandrock, are innumerable. Clay and oxide 



VOL. II. D S 



