DIFFERENCE BETWEEN SHALE AND SLATE. 115 



may have taken place since those days, one is very evident : 

 the great mass of mud has been upheaved until it stands 

 almost on end, and, consequently, though it is quite plain 

 that the layers were once in a horizontal position, they are 

 now some of them almost 

 vertical. They split off very 

 readily along the lines of 

 bedding, as one would ex- 

 pect them to do, for each 

 layer of mud would natu- 

 rally be slightly hardened 

 before the succeeding one 

 was deposited; but the fact 

 of the rock thus splitting 

 shows that, though of the 

 same age and composi- 

 tion as some of the best 



roofing-slates in the world, it is, after all, not true slate, 

 but shale (Fig. 22), for true slate splits, not along the lines 

 of bedding, but at some angle, often at right angles, to them, 

 and has clearly undergone some change which the shale 

 has escaped. 



Any fossils, such as shells, which occur in shales, are 

 found lying unaltered in shape, with their flat sides parallel 

 to the bedding, just as they would naturally have dropped 

 through the water. When they occur in slate, on the other 

 hand, they are distorted, and stand almost or quite on end, as 

 they would do if the mud enclosing them had been subjected 

 to great pressure from the sides. 



A mass of clayey mud, containing grains of sand, 



Fig. 22. SLICE OF SHALE, SEEN 

 UNDER THE MICROSCOPE AND 

 HIGHLY MAGNIFIED. 



