32 fiature's 



difference between slate and shale, for in- 

 stance, is not one of ingredients, but of the 

 process by which the ingredients are put to- 

 gether. All of the sedimentary rocks are 

 formed by a deposit of sediment from the 

 water on the bottom of the ocean. At one 

 period the floods have brought down a certain 

 kind of material in greater profusion than at 

 others, and this is deposited in thin layers, 

 and as it hardens there will be seams in it and 

 the stratifications will be differently colored, 

 the color depending upon the deposit at any 

 particular time. 



A bed of shale, like a bed of coal, has lines 

 of cleavage in it, and if it is examined under 

 a microscope it will be found that the sedi- 

 mentary particles, like the twigs and leaves in 

 the coal veins, lie with their longest dimen- 

 sions in line with the plane of cleavage. 

 Shale in color looks like slate, and an analysis 

 of the material of which it is formed shows 

 that shale and slate are both made from the 

 same. There is, however, a structural differ- 

 ence between the two which is very peculiar 

 and very interesting. The slate is ordinarily 

 a denser material and the lines of cleavage are 

 often at right angles with those that we find 

 in ordinary shale. 



A slab of shale will be of a uniform color on 

 any one line of cleavage. The color may 

 change at the next line, and generally does, to 



