182 STRUCTURAL GEOLOGY. 



in recent sediments, such as ripple-marks, rain-prints, 

 sun-cracks, foot-prints of animals, etc., are also found in 

 the hardest stratified rocks. In a word, it may be said 

 that every marlc or peculiarity which has been observed in 

 recent sediments has been found also in stratified rocks. 



We may assume, then, as certain that stratified rocks 

 are sediments formed originally at the bottom of seas, 

 lakes, rivers, etc., and that when we find them far in the 

 interior of continents and high up the slopes of mountains 

 we have indubitable evidence of great changes of level. 



Stratified rocks are all deposits in water. Sandstones 

 and shales are the debris of erosion, and are therefore 

 mechanical deposits; and these rocks are often called 

 fragmental rocks, because they are made up of the frag- 

 ments of previous rocks. Limestones, on the other hand, 

 are either organic or chemical deposits. Again, sand- 

 stones, grits, and conglomerates are formed by violent 

 action, and they indicate either rapid currents or exposed 

 shores ; shales indicate quiet seas or bays ; limestones^ 

 open seas. 



We have already seen (page 27 et seq.) that sediments 

 are transported soils, and (page 10) that soils are disinte- 

 grated rocks. Now, we see that stratified rocks are con- 

 solidated sediments. We have here an example of a per- 

 petually recurring cycle of changes : rocks are decomposed 

 into soils, soils are carried and deposited as sediments, 

 sediments are again consolidated into rocks, to be raised 

 into land-surfaces, and again disintegrated into soils and 

 so the cycle goes round. 



The cause of consolidation is sometimes only the 

 pressure of great thickness of sediment ; sometimes the 

 same, aided by gentle heat ; sometimes there is a distinct 

 cementing substance, the most common being lime car- 

 bonate and silica. When there is a cementing substance, 

 the process is often rapid, and may be observed ; as, for 

 example, in the formation of coral rock. But in other 



