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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. Arenicolites linearis, Hall. Arenig beds, Stiper-Stones. a. Didymograpsus geminus, Hisinger, sp.
When the term Silurian was given by Sir R. Murchison, in 1835, to the whole series, he considered the Stiper-Stones as the base of the Silurian system, but no fossil fauna had then been obtained, such as could alone enable the geologist to draw a line between this member of the series and the Llandeilo flags above, or a vast thickness of rock below, which was seen to form the Longmynd hills, and was called "unfossiliferous graywacke."
Next in the descending order are the shales and sandstones in which the quartzose rocks called Stiper-Stones in Shropshire occur. I have seen similar burrows now made on the retiring of the tides in the sands of the Bristol Channel, near Minehead, by lob-worms which are dug out by fishermen and used as bait.
The formations below the Arenig or Stiper-stones group are treated of in the next chapter, when the "Primordial" or Cambrian group is described. LUDLOW FORMATION: a. Upper Ludlow beds: 780. b. Lower Ludlow beds: 1,050. WENLOCK FORMATION: a. Wenlock limestone and shale and b. Woolhope limestone and shale, and Denbighshire grits: above 4,000. Lower Llandovery: 600-1,000.
This Etage C or primordial zone proved afterwards to be the equivalent of those subdivisions of the Cambrian groups which have been above described under the names of Menevian and Lingula Flags. The second fauna tallies with Murchison's Lower Silurian, as originally defined by him when no fossils had been discovered below the Stiper-Stones.
Wenlock Formation, Corals, Cystideans and Trilobites. Llandovery Group or Beds of Passage. Lower Silurian Rocks. Caradoc and Bala Beds. Brachiopoda. Trilobites. Cystideae. Graptolites. Llandeilo Flags. Arenig or Stiper-stones Group. Foreign Silurian Equivalents in Europe. Silurian Strata of the United States. Canadian Equivalents. Amount of specific Agreement of Fossils with those of Europe.
It was afterwards found that a large portion of the slaty rocks of North Wales, which had been considered as more ancient than the Llandeilo beds and Stiper-Stones before alluded to, were, in reality, not inferior in position to those Lower Silurian beds of Murchison, but merely extensive undulations of the same, bearing fossils identical in species, though these were generally rarer and less perfectly preserved, owing to the changes which the rocks had undergone from metamorphic action.
To such rocks the term "Cambrian" was no longer applicable, although it continued to be appropriate to strata inferior to the Stiper-Stones, and which were older than those of the Lower Silurian group as originally defined.
BALA AND CARADOC BEDS, including volcanic rocks: 12,000. LLANDEILO FLAGS, including volcanic rocks: 4,500. ARENIG OR STIPER-STONES GROUP, including volcanic rocks: above 10,000.
We have already seen that in the Arenig or Stiper-Stones group, where the species are distinct, the genera agree with Silurian types; but in these Tremadoc slates, where the species are also peculiar, there is about an equal admixture of Silurian types with those which Barrande has termed "primordial."
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