180 STRUCTURAL GEOLOGY. 



of water (page 27). In a word, the mass exposed on a 

 cliff or in a quarry, or any large section of stratified rock, 

 is seen to be divided by parallel planes into thick beds of 

 different kinds of materials, as sandstone, limestone, etc., 

 and each of these, probably, into thinner beds, differing 

 perhaps in grain or color, and finally these again into thin 

 sheets, produced by the sorting of material. Now, the 

 larger beds are called strata, the subdivisions of different 

 color or grain, layers, and the lines of sorted materials are 

 lamincB. These terms are loosely used, but always in the 

 order mentioned, and the word lamina is always used to 

 signify the marks of water-sorting. Now, the structure 

 we have described is called stratification, and such rocks 

 stratified rocks. 



Extent and Thickness. Stratified rocks cover at 

 least nine tenths of the land-surface, and even where they 

 do not occur it is only because they have been removed 

 by erosion or else covered by igneous rocks. Since, as we 

 shall see presently, stratified rocks were formed at the 

 bottom of the water, it is evident that there is no portion 

 of the earth which has not been at some time covered by 

 the sea. The extreme thickness of these rocks is proba- 

 bly ten to twenty miles ; the average thickness is certainly 

 several thousand feet. 



Principal Kinds. As defined above, stratified rocks 

 fall naturally into three great groups : 1. Arenaceous or 

 sand-rocks; 2. Argillaceous or clay-rocks; and, 3. Cal- 

 careous or lime-rocks. These may be either in a soft or 

 in a stony condition. 



The sand-rocks, in their soft or incoherent condition, 

 are beds of sand, gravel, and pebbles or shingle. In their 

 coherent or stony condition they are sandstones, grits, 

 and conglomerates. Breccias differ from conglomerates 

 only in having the fragments angular instead of rounded. 

 They consist of rubble, instead of pebbles, cemented 

 together. 



