MACKIE — ON TUli BOTTOM-ROCKS. 
18:$ 
as tho slates of Llaobcnia aud Baugor were derived, have long since 
subsided beneath the waves. 
Passing the Menai Straits, towards the west flanks of the moun- 
tainous range of Snowdon, we find " huge buttresses of very ancient 
grit, schist, slate, aud sandstone," of Cambrian date. 
But, after all, the best Bz'itish example of lowest sedimentary or 
" bottom-rocks '* occncs at tho Longmynd mountains, in the typical 
region of " Siluria," Shropshire, Aud there and at Bray Head only 
(with one or two isolated exceptions, referred to subsequently) have 
any traces of fossils as yet been found. 
I remember, many years since, seeing the Longmynds, when I knew 
very little about Geology, and nothing at all of the history of those 
hills, and thinking, as I passed them, what old hills they looked. Tho 
late Pi'ofessor Edward Forbes and others have expressed the same 
feeling ; and certainly there is something very remai'kable in their 
appearance. The valley to the west of them is bounded by some low 
hills of micaceous schist, ranging along the base of a craggy ridge of 
trap mountains, of which the Wrekin forms the northern extremity, 
and continued on the south side of the Severn by those of Acton- 
Bui-nell, Frodesley, the Lawley, Caer Caradoc, and Hope-Bo wdler. These, 
like the Wrekin, have the longest diameter from north-east to south- 
west, and rise very abruptly, at an angle of 60°, from the plain below. 
The vale in which Church Stretton is situated separates the trap moun- 
tains from the remarkable mass of hills called the Longmynds, which 
gradually rise to the height of 800 feet, and then with a level and 
nearly unvarying summit stretch for several miles towards Bishop's 
Castle. A peculiar squareness seems to characterize these mountains, 
and from Stretton Vale, whence three or four series of hills are seen 
rising one above another, this feature is particularly apparent. The 
individual mountains are generally separated from each other by a 
narrow deep glen, traversed in its length by a small stream, sometimes 
foaming, in cascades over rugged ridges, aud sometimes more gently 
flowing beneath overhanging woods. The Longmynds for the most 
part are covered with heath aud a short grass that furnishes exteu- 
Bive pasturage for numei'ous flocks of sheep ; and from their flanks 
brooks and streamlets break out and flow northward into the plain 
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