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Show 210 ISLAND LIFE. [PART I. Silurian formation is divided into Upper and Lower Silurian, each characterised by a distinct set of fossil remains, and the Upper Silurian again consists of a large number of separate beds, such as the Wenlock Limestone, the Upper Llandovery Sandstone, the Lower Llandovery Slates, &c., each usually characterised by a difference of mineral composition or mechanical structure, as well as by some peculiar fossils. These beds ·and formations vary greatly in extent, both above and beneath the surface, and are also of very various thicknesses in different localities. A thick bed or series of beds often thins out in a given direction, and sometimes disappears altogether, so that two beds which were respectively above and beneath it may come into contact. As an example of this thinning out, American geologists adduce the Palreozoic formations of the Appalachian Mountains, which have a total thickness of 42,000 feet, but as they are traced westward thin out till they become only 4,000 feet in total thickness. In like manner the Carboniferous grits and shales are 18,000 feet thick in Yorkshire and Lancashire, but they thin out southwards, so that in Leicestershire they are only 3,000 feet thick; and similar phenomena occur in all strata and in every part of the world. It must be observed that this thinning out has nothinO' to do with denuda- • 0 twn (which acts upon the surface of a country so as to produce great irregularities of contour), but is a regular attenuation of the layers of rock, due to a deficiency of sediment in certain directions at the original formation of the deposit. Owina to this thinning out of stratified rocks, they are on the whole of far less extent than is usually supposed. When we see a geological map showing successive formations following each other in lona • 0 Irregular belts across the country (as is well seen in the case of the Secondary rocks of England), and a corresponding section ~how~ng each bed dipping beneath its predecessor, we are apt to Imagme that beneath the uppermost bed we should find all the others following in succession like the coats of an onion. But this is far from being the case, and a remarkable proof of the narrow limitation of these formations has been recently obtained by a bo~ing at Ware through the Chalk and Gault Clay, which latter Immediately rests on the Upper Silurian Wenlock CHAl'. X.] THE EARTH'S AG K 211 Limestone full of characteristic fossils, at a depth of only 800 feet. Here we have an enormous gap, showing that none of earlier Secondary or late Palreozoic formations extend to this part of England, unless indeed they had been all once elevated and entirely swept away by denudation.! But if we consider how such deposits are now forming, we shall find that the thinning out of the beds of each formation and their restriction to irregular bands and patches, is exactl; what we should expect. 'l'he enormous quantity of sediment continually poured into the sea by rivers, gradually subsides to the bottom as soon as the motion of the water is checked. All the heavier material must be deposited near the shore or in those areas over which it is first spread by the tides or currents of the ocean; while only the very fine mud and clay is carried out to considerable distances. Thus all stratified deposits will form most quickly near the shores, and will thin out rapidly at greater distances, little or none being formed in the depths of the great oceans. This important fact was demonstrated by the specimens of sea-bottom examined during the voyage of the Challenge?·, all the " shore deposits " being usually confined within a distance of 100 or 150 miles from the coast, while the ' ' deep-sea deposits" are either purely organic, being formed of the calcareous or siliceous skeletons of globigerinre, radiolarians, and diatomacere, or are clays formed of undissolved portions of 1 The following statement of the depths at which the Palroozoic formations have been reached in various ·localities in and round London was given by Mr. H. B. Woodward in his address to the Norwich Geological Society in 1879:- Deep Wells through the Tertim·y and O·etaceous Formations. Harwich .................. at 1,022 feet reached Carboniferous Rock. Kentish •rown ........... , 1,114 , , Old Red Sandstone. Tottenham Court Road, 1,064 , , Devonian. Blackwall ............... , 1,004 , , Devonian or Old Red Sandstone. Ware ..................... , 800 , , Silurian (Wenlock Shale). ' 'V e thus find that over a wide area, extending from London to Ware and Harwich, the whole of the formations from the Oolite to the Permian are wanting, the Cretaceous resting on the Carboniferous or older Palroozoic rocks; and the same deficiency extends across to Belgium, where the Tertiary beds are found resting on Carboniferous at a depth of less than 400 feet. p 2 |