LITHOLOGY. 
53 
gether so as to form a sort of pudding stone or indurated gravel, 
of wliicli Bramley Fall, especially the coarse grained variety, is 
a good example ; if these various substances be exposed to the 
same causes of decomposition, they will resist them unequally, 
those parts of the stone which decompose most readily being re- 
moved, leave the durable portions exposed, so as to be more 
easily affected by mechanical forces, such as the freezing and 
expanding of water after being absorted by the pores of the stone, 
occasioning a disruption of the external particles ; a constant 
succession of these chemical and mechanical operations, will dis- 
integrate a mass of stone, composed of various substances, in 
much less time than if the same bulk were formed of one uniform 
homogenous material, if that one material even were the most 
decomposable portion of the conglomorate to which allusion has 
just been made. 
After having examined the component parts of a stone, the next 
consideration must be the condition of those parts to form the 
aggregate; in all specimens of genuine sandstone, the grains are 
infinitely more dm'able than the cement which unites them, and 
in some cases, notwithstanding the cement is of the best quality 
it may be so scantily supplied, that the grains are scarcely at- 
tached to each other, consequently such stone must be extremely 
friable, and will suffer from all kinds of mechanical forces. The 
great portion of the mass of all sandstone is made up of quartzy 
or siliceous grains, if the substance that cements the grains to- 
gether be also siliceous the stone will most probably be very 
durable ; but w^e frequently find siliceous grains cemented with ar- 
gillaceous, calcareous, ferruginous or other matters, each of which 
has more or less affinity for the Silica, and of course the grains 
are more or less attached to each other. Those cements which 
partake most of the character of iron or clay, are most likely to 
separate from the grains of quartz; consequently, a sandstone 
thus composed, will, in all probability, soon yield to atmospheric 
influences, because the oxygen of the atmosphere produces con- 
