16 



DIAGONAL OR CROSS STRATIFICATION. 



[Ch. II. 



water more than a third of the weight which they have in air, the spe- 

 cific gravity of rocks being in general as 2i 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 de- 

 posits 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 irregularities. 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 out, and allows the beds which were previously 

 above and below it to meet. If the materials are coarse, as in grits and 

 conglomerates, the same beds can rarely be traced many yards without 

 varying in size, and often coming to an end abruptly. (See fig. 2.) 



Fig. 2. 



Section of strata of sandstone, grit, and conglomerate. 



Diagonal or Cross Stratification. — 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 



Fig. 3. 



Section of sand at Sandy Hill, near Biggleswade, Beetfonlshire. 

 Height 20 feet. (Green-sand formation.) 



planes of stratification. To this diagonal arrangement the name of 

 " false or cross stratification" has been given. Thus in the annexed sec- 

 tion (fig. 3) we see seven or eight large beds of loose sand, yellow and 



