76 
DECAY IN NATURE. 
April, 1892. 
distance of about ten miles, but in the course of ages this 
broad band of sandstone has been worn and washed away, 
leaving the wide and shallow valley of the Tern. Let us 
assume that the escarpments have been retreating towards 
the north at the rate of one inch in a century, and that they 
have travelled only half the distance between the Wrekin and 
their present position. This would give us over thirty 
millions of years for the carving out of the valley of the Tern. 
Some of our Shropshire hills are composed of more 
durable material than the sandstone of the northern eleva¬ 
tions. Tiie chain of hills running from Lilleshall to Church 
Stretton, of which the Wrekin and Caer Caradoc are the 
most conspicuous summits, is mainly formed of volcanic 
ashes and lavas of some firmness of texture. These rocks 
decay with comparative slowness ; but a little experience with 
a hammer convinces us that the solidity is more apparent 
than real. Strike a block of the Wrekin lava a smart blow, 
and it often shivers up into a multitude of small fragments. 
This weakness is due to the cracks (joints) which penetrate 
the rock. Sometimes the cracks run in one direction, so that 
the rock splits up into slabs ; sometimes in two directions, 
when the fragments may be wedge-like; sometimes in 
three or more directions, so that the rock breaks up into 
irregular pieces. What the hammer can do in an instant, 
water and frost can do quite as effectually, though they work 
more slowly. In this way, our hard Wrekin lavas and ashes, 
when exposed to the weather, shiver up at the surface into 
loose fragments. 
The granite of the Ercal, near Wellington, is a singular 
instance of how a rock which is nroverbiallv hard mav 
«. «/ •/ 
become one of the frailest. In this case, the cracks which 
penetrate the rock are so close together that the granite is sliced 
up into very small pieces. Under the action of the weather 
these fall away from each other, and a fine gravel is formed, 
which is shovelled into carts, and used in the making of 
garden paths. 
One of the most durable of rocks is quartzite. It consists 
of grains of quartz cemented together by quartz, Being 
composed of a mineral which is incapable of decay—for 
quartz cannot be resolved into simpler elements by natural 
forces—it can only be affected by causes which act upon it 
mechanically. It is usually penetrated by cracks which are 
not very close together, and under atmospheric influences it 
weathers into blocks of some magnitude. Owing to these 
causes it perishes but slowly. A band of quartzite forms the 
culminating ridge of the Stiper Stones, the shales below it 
and the flags above it being more destructible, and being 
