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 again 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 marl that the rock- 
salt has hitherto been found in England. It also 
contains beds of gypsum. In Poland, extensive 
beds of iron ore are found in this marl ; 27 furna- 
ces, affording annually 560 quintals of metal. Fos- 
sils are rare in this deposite, with the 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 . chemical 
agents, and rapidly decomposing in consequence, 
any one who passes up the Hudson River may be 
