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the thickness of our stratified rocks. The varying thicknesses and characters of our formations may, in my opinion, be frequently accounted for by considering that the changes in physical conditions did not take place uniformly, but that a clayey, a sandy, or a calcareous formation may have occupied a longer period in its deposition in one locality than in another, while the succeeding formation may tell of conditions which locally were of shorter duration. An estimate of 55,000 feet seems to me to be a reasonable computation for the total thickness of strata represented in England and Wales.
We cannot, however, be exact in our measurements, nor is it possible to calculate with certainty the amount of strata unrepresented in our Islands. Professor Ramsay, indeed, would lead us to believe that the strata unrepresented were as great as, perhaps greater than, those preserved to us. It is true that we know but little of the very earliest or Pre-Cambrian strata: between them and the Cambrian there is evidence of a great break. Again there is considered to be an unconformability between the Cambrian and Silurian strata of Sedgwick; there is a great palæontological and physical break between the Chalk and the Tertiaries; and between our Eocene and Pliocene there is a break, which is partly bridged over by the Miocene deposit of Bovey Tracey. Locally, of course, there are many unconformabilities, but I believe that on the whole the succession of strata in England and Wales is far more complete than is generally taught, and that even the breaks above indicated, excepting cnly that between the presumed Laurentian and Cambrian, do not necessarily indicate any very great interval of time. And I should consider that the whole time which has elapsed from the beginning of the formation of the known strata might be fairly represented by estimating their total thickness at 75,000 feet, and supposing them to have been deposited uniformly and without break.
Sir William Thomson has calculated that the sun has probably not illuminated the earth for 100,000,000 years, and almost certainly not for 500,000,000 years. This gives a limit to our estimates of time which, notwithstanding its vagueness, we should be careful not to overstep.
The question has naturally arisen as to whether this estimated period of time is sufficient for all geological changes. Taking 100,000 feet as a full allowance for the total thickness of stratified rocks containing traces of life, Professor Huxley has pointed out that, restricting the time to 100,000,000 years, the deposits may be estimated to have taken place at the rate of of a foot, or of an inch If we take 75,000 feet as an estimate, we may,
for a similar period, calculate the rate of deposit at about 100
Whatever estimate prove correct, however, there is no doubt that geological time has been inconceivably great' -every one will admit this, because the mind cannot realise the amount-it eludes the grasp of both memory and imagination.'
The following Table shows the classification adopted in this book, the names of authors being appended to the terms which they have introduced:
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 we 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 Paleozoic 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 Paleozoic 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 Paleozoic 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,