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. 2. 



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 grits and con- 

 glomerates, the same beds can rarely be traced many yards 

 without varying in size, and often coming to an end abruptly. 

 (See Fig. 2.) 



There is also another phenomenon of frequent occurrence. 

 We find a series of larger strata, each of which is composed of 

 a number of minor layers placed obliquely to the general planes 

 of stratification. To this diagonal arrangement the name of 



