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Eurypterus, etc., others, called trilobites, as Trinucleus, Asaphus, Calymene, Phacops; and many genera of corals, as Heliolites, Favosites, Calceola; and crinoids, as Actinocrinus, Platycrinus, Pentremites. The plants of this series are chiefly cryptogams, Ferns, Lycopodiaceæ, and Calamites. Graptolites also are characteristic of Palæozoic strata.
LEWISIAN OR FUNDAMENTAL GNEISS. (Murchison.)
The term Laurentian was given by Sir William Logan to a series of highly contorted gneissic rocks which occur in Canada, in the country drained by the St. Lawrence, and constitute the oldest known sedimentary rocks in the world. Their thickness is estimated at 30,000 feet. Zones of altered limestone included in the series have yielded traces of foraminiferal structure, and the name Eozoon Canadense has been applied by Dr. J. W. Dawson to this the oldest evidence of life yet discovered.2
The existence of Laurentian rocks in England and Wales, or of English rocks equivalent to the oldest rocks of America, has not been definitely determined; nor is it probable that, in comparing the rocks of these two countries, we can ever do more than state that certain series of strata are homotaxeous, or in other words that they occupy the same relative position in regard to the succession of life in the two areas. The Laurentian rocks, where developed in North Britain, are unconformably overlaid by the Cambrian rocks.
In certain localities in England and Wales there is
1 This term was taken from the island of Lewis, the largest of the Hebrides, where the rocks were first described by Murchison.
2 The occurrence of Graphite in these rocks is considered as evidence of a yet earlier vegetation having existed.
evidence of the presence of rocks of Pre-Cambrian age, and therefore it is well to mention some of the opinions that have been expressed concerning them.
England. Dr. Holl has stated his opinion that the gneissic rocks of Malvern may agree in point of age with the Laurentian series of Canada-that they are the relics of an old Pre-Cambrian continent. Murchison however considered them to be metamorphosed Cambrian strata.
The oldest rocks of Charnwood Forest may in the opinion of both Prof. Ansted and Dr. Holl belong to the Laurentian series. These rocks according to the former geologist (speaking generally) include imperfect and true slates and claystones passing into claystone porphyry and hornblendic porphyry, and so into granitic gneiss, granite and syenite. The slates are quarried for roofing purposes near Swithland. In Charnwood Forest, at Whittle Hill, whetstones are obtained called Whittle Hill Hones, which are said to be among the best substitutes for the Turkey oilstone.
There is a rock too at the Abberley Hills which the Rev. W. S. Symonds has suggested may be of Laurentian age.
Wales. Some of the crystalline rocks of Anglesea, Holyhead, and the adjacent parts of Caernarvonshire; also the syenitic rocks of St. David's, are regarded as of Pre-Cambrian age, including (according to Mr. Hicks) some old quartz-conglomerate and dark green shales.
This term is derived from Cambria, the old name for Wales, and as a group it is equivalent in part to the Cumbrian' series of the Lake District.
About the year 1834 the name Cambrian was given by Sedgwick to the great mass of the slaty rocks and limestones in North Wales, which at the time were considered to be older than the Silurian rocks of Murchison.
The subsequent researches of Sedgwick enabled him to fix the boundary line of the Cambrian and Silurian rocks at the base of the May Hill Group, which the further observations of geologists have tended to confirm, as a line wellmarked by a physical break. Murchison, however, who had included the Caradoc and Llandeilo beds in his Silurian system, but had misplaced them in his typical section, did not concede these strata to the Cambrian; and finding that his system had no definite base on which to rest, took in from time to time group after group of the underlying series, and had to prove at each step that as yet no break had been found in the series, till at length he got down to the lowest Cambrian.'
There is therefore some confusion in the different classifications adopted by geologists, some upholding that of Sedgwick, some (including the officers of the Geological Survey in their publications) adopting that of Murchison; while others again have endeavoured to effect a compromise, and have drawn the line between Cambrian and Silurian about midway in the debateable ground, or in the Lower Silurian.
As the opinions most deserving of consideration differ
1 See Sedgwick, Palæozoic Rocks, and Catalogue of Cambrian and Silurian Fossils. Cambridge. Also T. Sterry Hunt, History of the Names Cambrian and Silurian; Professor T. McK. Hughes, Discussion at Meeting of Geol. Soc., December 2, 1874; Rep. Brit. Assoc., 1875.
on this somewhat tender subject of classification, I cannot do better than indicate those of both Sedgwick and Murchison. At the same time, for the sake of convenience, I shall arrange the subject-matter according to that of the former geologist, the results of whose early researches amongst the older Palæozoic rocks of North Wales have been shown to be in all essential points correct, but have scarcely been adequately acknowledged.
It is perhaps needless to observe that the names Cambrian and Silurian are mere terms of convenience in classification : one name might indeed be applied to the whole series of rocks, but as we have no reason to doubt that the whole history of the earth was a series of regular changes, so the terms of classification, dividing the history into chapters and paragraphs, are useful divisions that assist the memory and mark out the leading modifications that have taken place over any given area. The divisions between Cambrian and Silurian strata mark no great change in the life history, and yet the classification now adopted is convenient as marking the greatest physical break yet noticed in the series in England and Wales.
The following Table shows the classification of the strata :
The total maximum thickness of the Cambrian strata is estimated at upwards of 30,000 feet in Wales, and at about 20,000 feet in the Lake district. But it must be remembered that owing to the disturbances and changes the beds have undergone, such estimates are very hypothetical.
The Cambrian rocks have, on the whole, been formed in shallow seas, but there are some evidences of tolerably deep
The Lower Cambrian rocks are often subdivided geographically, or according to the several tracts over which they are exposed. Two well-marked groups are, however, now established :
Longmynd, or Harlech Group.
LONGMYND, OR HARLECH GROUP.
The rocks of this group consist of a series of grey, purple, and red flaggy sandstones, conglomerates and shaly beds, having a thickness estimated at 4,000 feet in South Wales, and supposed to be over 8,000 feet in North Wales. Remains of Sponges, Annelids, Pteropods, Polyzoa, Brachiopods, such as Lingulella and Obolella; also of Entomostraca, and Trilobites of the genera Conocoryphe, Paradoxides, Microdiscus and Plutonia, have been determined in these strata, mainly through the researches of Mr. Hicks.
These indicate the marine origin of the strata in which they occur. Professor Ramsay has suggested that some of the red or purple beds which are unfossiliferous, may have been deposited in inland waters, or lacustrine areas subject to occasional influxes of the sea.