Beneath the black slates above described of the Llandeilo formation, Graptolites
are still found in great variety and abundance, and the characteristic genera of
shells and trilobites of the Lower Silurian rocks are still traceable downward,
in Shropshire, Cumberland, and North and South Wales, through a vast depth of
shaly beds, in some districts interstratified with trappean formations of
contemporaneous origin; these consist of tuffs and lavas, the tuffs being formed
of such materials as are ejected from craters and deposited immediately on the
bed of the ocean, or washed into it from the land. According to Professor
Ramsay, their thickness is about 3300 feet in North Wales, including those of
the Lower Llandeilo. The lavas are feldspathic, and of porphyritic structure,
and, according to the same authority, of an aggregate thickness of 2500 feet.
ARENIG OR STIPER-STONES GROUP (LOWER LLANDEILO OF MURCHISON).
(FIGURE 563. Arenicolites linearis, Hall. Arenig beds, Stiper-Stones.
a. Parting between the beds, or planes of bedding.)
(FIGURE 564. Didymograpsus geminus, Hisinger, sp.
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