OCR Text |
Show 28-1 IMPERFECTION OF THE CHAP. IX. are the result and measure of the degradation which the earth's crust has elsewhere suffered. And what an amount of degradation is implied by the sedimentary deposits of many countries! Professor Ramsay has given me the maximum thickness, in most cases from actual measurement, in a few cases from estimate, of each formation in different parts of Great Britain ; and this is the result :- Feet. Palreozoic strata (not including igneous beds) .. 57,154 Secondary strata . . . . 13,190 Tertiary strata 2,240 -.making altogether 72,584 feet ; that is, very nearly thirteen and three-quarters British miles. Some of these formations, which are represented in England by thin beds, are thousands of feet in thickness on the Continent. Moreover, between each successive formation ' ' we have, in the opinion of most geologists, enormously long blank periods. So that the lofty pile of sedimentary rocks in Britain, gives but an inadequate idea of the time which has elapsed during their accumulation; yet what time this must have consumed ! Good observers have estimated that sediment is deposited by the great Mississippi river at the rate of only 600 feet in a hundred thousand years. This estimate may be quite erroneous; yet, considering over what wide spaces very fine sediment is transported by the currents of the sea, the process of accumulation in any one area must be extremely slow. But the amount of denudation which the strata have in many places suffered, independently of the rate of accumulatio~ of the degraded matter, probably offers the . best evrdence of the lapse of time. I remember havrng been much struck with the evidence of denudation, when viewing volcanic islands, which have been CHAP. IX. GEOLOGICAL RECORD. 285 w~rn by the waves and pared all round into perpendicular cliffs of one or two thousand feet in height ; for the gentle slope of the lava-streams, due to their formerly liquid state, showed at a glance how far the hard, rocky beds had once extended into the open ocean. The same story is still more plainly told by faults,-those great cracks along which the strata have been upheaved on one side, or thrown down on the other, to the height or depth of thousands of feet ; for since the crust cracked, the surface of the land has been so completely planed down by the action of the sea, that no trace of these vast dislocations is externally visible. The Craven fault, for instance, extends for upwards ,of 30 miles, and along this line the vertical displacement of the strata has varied from 600 to 3000 feet. Prof. Ramsay has published an account of a downthrow in Anglesea of 2300 feet ; and he informs me that he fully believes there is one in Merionethshire of 12,000 feet ; yet in these cases there is nothing on the surface to show such prodigious movements ; the pile of rocks on the one or other side having been smoothly swept a':ay. The ?onsideration of these facts impresses my mrnd almost rn the same manner as does the vain endeavour to grapple with the idea of eternity. I am tempted to give one other case, the well-known one of the denudation of the 'V eald. Though it must be admitted that the denudation of the Weald has been a mere trifle, in compal'ison with that which has removed masses of our palooozoic strata, in parts ten thousand feet in thickness, as shown in Prof. Ramsay's masterly memoir on this subject. Yet it is an admirable lesson to stand on the North Downs and to look at the distant South Downs ; for, remembering that at no great distance to the west the northern and southern escarpments meet and close, one can safely picture to |