98 UPPER SECONDARY ROCKS. 



comparatively short duration, succeeded by long in- 

 tervals of repose, during which the calcareous strata 

 were deposited. 



We need not enter upon a description of these 

 three formations of the red sandstone, because 

 their character and limits are not, as yet, very clear- 

 ly defined. This rock is often variegated in colour 

 and various in its texture. Sometimes it is com- 

 posed of large pebbles and fragments of other rocks 

 cemented together; and then Hgain it passes into 

 variegated marls. These are more generally of a 

 red colour, and cover the sandstone in beds of con- 

 siderable thickness, forming a stiff reddish loam, 

 with greenish or yellowish stripes, as in many parts 

 of Virginia. These beds are sometimes 500 feet in 

 thickness, and it is in this red mwrl that the rock- 

 salt has hitherto been found in England. It also 

 contains beds of gypsum. In Poland, extensive 

 beds of iron ore nre found in this marl ; 27 furna- 

 ces, affording annually 560 quintals of metal. Fos- 

 sils are rare in this deposiie, with tlie exception of 

 vegetable remains. In Swabia it is rich in both iron 

 and coal. It is now pretty well ascertained that the 

 red marl, as well as the sandstone, have been prin- 

 cipally formed by the disintegration of rocks of trap, 

 greenstone, syenite, and granular quartz : the iron of 

 the trap rocks communicating the red colour by 

 oxidation. 



Bakewell supposes that the argillaceous marls 

 have been principally formed from the trap rocks, 

 and the silicious sandstones from the granular 

 quartz rock. In proof of this, it is stated that the 

 red ground in the vicinity of the different trap rocks 

 in Devonshire is invariably composed of fragments 

 of these rocks, increasing in size as they are situ- 

 ated nearer the original formation. That trap rocks 

 are easily affected by mechanical and cliemical 

 agents, and rapidly decomposing in consequence, 

 any one who passes \w the Hudson River may be 



