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PA L Æ O ZO I C.
LAURENTIAN-CAMBRIAN. The term Palæozoic, derived from words which mean ancient life,' is applied to the oldest known rocks that form the crust of the earth. They are the ‘Primary' rocks with which the Geologist has to deal; and although this latter term is now but little used, as
are unable to state positively that the oldest rocks known to us were those first formed, yet it is not altogether inappropriate, as in the Primary or Palæozoic age we shall always include the earliest known rocks and those formed up to the close of our Coal period. The term is one of convenience rather than one defining any very great physical or palæontological break. Many authorities would place the Permian rocks among those of Palæozoic age, but they seem in their sedimentary character to be so closely allied to the newer Triassic rocks, that it appears to me best to adopt the old name Poikilitic as a comprehensive term for both Permian and Trias; and to place this great group at the base of the Mesozoic or Secondary strata, because the Triassic rocks are so inseparably connected through the Rhætic beds with the Lias and Oolites.
The terms Azoic (without life) and Eozoic (dawn of life) are now but little used, because by the discovery of organic structure in the very oldest known rock, the first has become erroneous when applied to the earliest stratified aqueous formations, and the second is really an unnecessary addition to our nomenclature.
In studying the rocks of Palæozoic age, it is well to
remember what a vast series of strata they constitute, although at the same time they do not enter so largely as the Secondary and Tertiary rocks into the superficial structure of our country. The total thickness of the Palæozoic series may be roughly estimated at 45,000 feet, whereas the total thickness of the newer strata cannot exceed 10,000 feet. Moreover in the latter, the several subdivisions are well marked, and as a rule easily recognised, for they have not suffered the alteration that the older rocks have. The number of main subdivisions in the Palæozoic series is roughly similar to those identified and mapped out in the Secondary strata. But it must be observed that the number of minor subdivisions increases as we ascend the geological scale and reach the more recent deposits. In the lower divisions of the Palæozoic rocks the rarity and frequent obscurity of the organic remains, the difficulty in identifying the strata, and often of determining the true dip, have no doubt considerably reduced the number of geologists who have particularly studied these rocks. The Carboniferous rocks as a rule are readily recognised, but the exact age of the Devonian strata is a subject still under discussion.
The Palæozoic rocks possess generally a more crystalline and slaty structure than the newer strata ; they are also more highly inclined and faulted, more frequently contorted, and more affected by igneous intrusion and metamorphism. They generally occupy more elevated ground, forming lofty hills and mountains, and are comparatively destitute of vegetation. They are the principal repositories, in England and Wales, of metallic wealth, and of coal.
The Palæozoic series is marked by the presence of fishes with heterocercal (unequal-lobed) tails ; of mollusca belonging to the genera Orthoceras, Goniatites, Leptæna, Spirifer, Productus, Orthis, Pentamerus, Strophomena; numerous extinct forms of crustacea of great size, called Pterygotus, Eurypterus, etc., others, called trilobites, as Trinucleus, Asaphus, Calymene, Phacops; and many genera of corals, as Heliolites, Favosites, Calceola; and crinoids, as Actinocrinus, Platyerinus, 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.?
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.
Fig. 1. --General Section of Malvern.
(Prof. J. Phillips.)
a. Gneiss (Laurentian ?).
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-Canubrian 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
See Sedgwick, Paleozoic 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.