The researches of geologists have discovered but scanty traces of
organic remains in the rocks which form the base of this system in
England. _Arenicolites_, or worm-tracks and burrows, have been found in
Shropshire, by Mr. Salter, to occur in countless numbers through a mile
of thickness in the Longmynd rocks; and others were discovered by the
late Dr. Kinahan in Wicklow. In Ireland, in the picturesque tract of
Bray Head, on the south and east coasts of Dublin, we find, in slaty
beds of the same age as the Longmynd rocks, a peculiar zoophyte, which
has been named by Edward Forbes _Oldhamia_, after its discoverer, Dr.
Oldham, Superintendent of the Geological Survey of India. This fossil
represents one of the earliest inhabitants of the ocean, which then
covered the greater part of the British Isles. “In the hard, purplish,
and schistose rocks of Bray Head,” says Dr. Kinahan,[34] “as well as
other parts of Ireland which are recognised as Cambrian rocks, markings
of a very peculiar character are found. They occur in masses, and are
recognised as hydrozoic animal assemblages. They have regularity of
form, abundant, but not universal, occurrence in beds, and permanence of
character even when the beds are at a distance from each other, and
dissimilar in chemical and physical character.” In the course of his
investigations, Dr. Kinahan discovered at least four species of
Oldhamia, which he has described and figured.
[34] Trans. Roy. Irish Acad., vol. xxiii., p. 556.
The Cambrian rocks consist of the Llanberis slates of Llanberis and
Penrhyn in North Wales, which, with their associated sandy strata,
attain a thickness of about 3,000 feet, and the Barmouth and Harlech
Sandstones. In the Longmynd hills of Shropshire these last beds attain a
thickness of 6,000 feet; and in some parts of Merionethshire they are of
still greater thickness.
Neither in North Wales, nor in the Longmynd, do the Cambrian rocks
afford any indications of life, except annelide-tracks and burrows. From
this circumstance, together with general absence of Mollusca in these
strata, and the sudden appearance of numerous shells and trilobites in
the succeeding Lingula Flags, a change of conditions seems to have
ensued at the close of the Cambrian period.
Believing that the red colour of rocks is frequently connected with
their deposition in inland waters, Professor Ramsay conceives it to be
possible, that the absence of marine mollusca in the Cambrian rocks may
be due to the same cause that produced their absence in the Old Red
Sandstone, and that the presence of sun-cracks and rain-pittings in the
Longmynd beds is a corroboration of this suggestion.[35]
[35] “On the Red Rocks of England,” by A. C. Ramsay. _Quart. Jour.
Geol. Soc._, vol. xxvii., p. 250.