. Elements of geology. Geology. PART 1. CHAPTER 11. 31 Original Horizontality Thinning out. sand and ashes c d the surface at e being quite level. It will be seen, that although the materials of the first layers have ac- commodated themselves, in a great degree, to the shape of the ground A B, yet each bed is thickest at the bottom. At first, a Y'lg, 1. great many particles would be car- ried by their own gravity down the steep sides of A and B, and others would afterwards be blown by the wind as they fell off the ridges, and would settle in the hollow, which would thus become more and more ef
. Elements of geology. Geology. PART 1. CHAPTER 11. 31 Original Horizontality Thinning out. sand and ashes c d the surface at e being quite level. It will be seen, that although the materials of the first layers have ac- commodated themselves, in a great degree, to the shape of the ground A B, yet each bed is thickest at the bottom. At first, a Y'lg, 1. great many particles would be car- ried by their own gravity down the steep sides of A and B, and others would afterwards be blown by the wind as they fell off the ridges, and would settle in the hollow, which would thus become more and more effaced as the strata accumulated from c to e. This level- ling operation may, perhaps, be rendered more clear to the stu- dent, by supposing a number of parallel trenches to be dug in a plain of moving sand, like the African desert, in which case the wind would soon cause all the signs of these trenches to disap- pear, and the surface would be as uniform as before. Now, water in motion can exert this levelling power on similar materials more easily than air, for almost all stones lose in water more than a third of the weight which they have in air, the specific gravity of rocks being in general as 2^ when compared to that of water, which is estimated at 1. But the buoyancy of sand or mud would be still greater in the sea, as the density of salt water exceeds that of fresh. Yet, however uniform and horizontal may be the surface of new deposits in general, there are still many disturbing causes, such as eddies in the water, and currents moving first in one and then in another direction, which frequently cause irregulari- ties. We may sometimes follow a bed of limestone, shale, or sandstone, for a distance of many hundred yards continuously ; but we generally find at length that each individual stratum thins Fig. Section of strata of sandstone, grit, and conglomerate. out, and allows the beds which were previously above and below it to meet. If the materials are course, as in g
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Keywords: ., bookauthorlyellcharlessir17, bookcentury1800, booksubjectgeology
