222 
STRUCTURE OF THE TITTERSTONE CLEE HILL. 
stream lias been deposited, so that—as is well seen in a 
section of a valley near Ontraigne, in the department of 
Ardeche, the columnar structure, which is vertical at the 
bottom of the valley becomes nearly horizontal along its 
sides. The columnar is not, however, the only form in which 
this rock is found. In many instances it assumes the appear¬ 
ance of spherical masses resembling cannon balls. I have 
observed this structure on a very large scale in Natal, where 
basalt exists through vast tracts of country. An example of 
the same may also be seen near the Breidden, where also you 
will find abundant specimens of porous cellular lava, which, 
when the almond-shaped cavities with which it abounds have 
been filled by the infiltration of some other material, is called 
amygdaloid. 
You have within the limits of Shropshire good opportu¬ 
nities of studying ancient volcanic phenomena. Corndon is 
an example of an extinct submarine volcano. All round that 
hill you find ridges of felspathic ash alternating with shale, 
telling us of a time when, just as in the present day, showers 
of stones and ashes are projected from a crater and spread 
out at the bottom of the sea. A period of rest then takes 
place, during which the shale (often fossiliferous) is deposited. 
The Breidden and the Clee Hills, on the other hand, are 
instance of intrusion ; and at Lincoln Hill, near Wellington, 
you may observe a fine example of a basalt dyke, a mass of 
molten rock having insinuated itself between the joints of 
the already deposited strata. 
And now a word as to the age and conditions of this 
outburst of volcanic matter. We are now looking westward 
across an undulating plain ; those successive ridges of hills 
which you see below us represent the various members of the 
Silurian formation, and bounding our view to the N.W. you 
see the Longmynd Hills, the representative of a still earlier 
epoch, viz., the Cambrian. All these were deposited many 
ages before the Devonian or Old Red which occupies the 
nearer ridges, and lastly, the Carboniferous, of which these 
hills are composed. Finding, then, as we do, that the basalt 
on which we now stand has spread out over the last-named 
stratum, the inference is inevitable that its age is subsequent 
to that of the coal period. That these hills, however, were, 
as has been, I believe, supposed by some, the site of an 
extinct volcano is very doubtful; and I quite agree with Mr. 
Yates, who has studied the subject from a practical point of 
view, that the evidence is rather in favour of an intrusion of 
molten matter between the strata, and that the more recent 
rocks which once overlaid the coal measures have been sub- 
