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Updated: June 25, 2025
As to the Cambrian, it is supposed by Sir Roderick Murchison that the fragment left in Sutherlandshire is about 3500 feet thick, and in Wales and the borders of Shropshire this formation may equal 10,000 feet, while the Silurian strata No. 3, difficult as it may be to measure them in their various foldings to the eastward, where they have been invaded by intrusive masses of granite, are supposed many times to surpass the Cambrian in volume and density.
But some clues are found in nature or in embryonic development, and, when the types do begin to be preserved in the rocks, we find the process of evolution plainly at work in them. We will therefore say a few words about the general evolution of each type, and then return to the geological record in the Cambrian rocks.
Wallace, on the other hand, seems more disposed to accept them, and, after considering Sir William's objections and those of Mr. Croll, puts the probable date of the beginning of the Cambrian deposits at only twenty-four million years ago.
He says: "While, however, there are not apparent signs of mechanical disturbances, during the long period that elapsed from the cooling of the earth's surface to the deposition of the Silurian and Cambrian systems, it is to be presumed that the internal igneous activity of the earth's crust was in full force, so that on the inner side of it, in obedience to the laws of specific gravity, chemical attraction, and centrifugal force, a great segregation of silica in a molten state took place.
Omitting structures whose organic nature has been questioned, there are left to mention a tiny seashell of one of the most lowly types, a DISCINA from the pre- Cambrian rocks of the Colorado Canyon, and from the pre-Cambrian rocks of Montana trails of annelid worms and casts of their burrows in ancient beaches, and fragments of the tests of crustaceans.
No remains, however, of them or of any vertebrate animal have yet been discovered in the Ordovician strata, rich as these are in invertebrate fossils, nor in the still older Cambrian; so that we seem authorised to conclude, though not without considerable reserve, that the vertebrate type was extremely scarce, if not wholly wanting, in those epochs often spoken of as "primitive," but which, if the Development Theory be true, were probably the last of a long series of antecedent ages in which living beings flourished.
Amid cheers that rent the welkin, responded to by answering cheers from a big muster of henchmen on the distant Cambrian and Caledonian hills, the mastodontic pleasureship slowly moved away saluted by a final floral tribute from the representatives of the fair sex who were present in large numbers while, as it proceeded down the river, escorted by a flotilla of barges, the flags of the Ballast office and Custom House were dipped in salute as were also those of the electrical power station at the Pigeonhouse and the Poolbeg Light.
If the eozoon Canadense, found in the laurentian slate of the Cambrian formation in North America, is really an organism and not an inorganic form, the earliest vestiges of animal life we can find are the rhizopodes or foraminifera; and these organisms belong to the lowest stage of life to that stage which forms a kind of undeveloped intermediate member between the vegetable and animal kingdom, Häckel's kingdom of the protista.
Not very long ago M. Barrande added another and lower stage, abounding with new and peculiar species, beneath the then known Silurian system; and now, still lower down in the Lower Cambrian formation, Mr Hicks has found South Wales beds rich in trilobites, and containing various molluscs and annelids.
As there is a very fine passage describing the Cambrian Hills by moonlight, we enclose herewith a condensed table showing the mean altitude of the moon for the month of December in the latitude of Wales. The character of Miss Plynlimmon we find to be developed in conversation with her grandmother, which we think you had better not read.
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