1 84 CONSOLIDATION OF SEDIMENTS 



(4) Consolidation by Lateral Pressure. — This is probably the 

 most widely acting and important agency of consolidation. 

 Though it acts so gradually and at such depths that we cannot 

 see it in operation, yet the inference is, none the less, a safe one. 

 We shall see later that very many of the stratified rocks are no 

 longer in the nearly horizontal position in which they were first laid 

 down, but have been folded and fractured through the operation of 

 great lateral pressures. The more intensely folded and compressed 

 any rock has been, the harder has it become, not only through the 

 mechanical pressure, but by the heat and the chemical changes 

 which such compression generates. In addition to this, we know 

 from experiment that loose materials may be consolidated by power- 

 ful compression. Certain exceptional rocks of very ancient date 

 are known, which are almost as incoherent as when first accumu- 

 lated, but these all retain their original horizontal position and have 

 not been compressed. It must not be supposed, however, that 

 only compressed sediments have become hard, for great areas 

 of scarcely disturbed rocks are found, which are perfectly solid 

 and firm ; here some other solidifying agent has been at work. 



There are certain other features in which the loose modern 

 sediments differ from the older and harder rocks, such as joints, 

 and cleavage which divides many rocks into thin plates, indepen- 

 dently of the planes of stratification. These may be shown, how- 

 ever, to be structures which the rocks have acquired after their 

 formation, and therefore need not be discussed here. 



The parallel is now complete between the sediments which we 

 may observe to-day in the process of accumulation, and the hard 

 stratified rocks which make up by far the largest part of the dry 

 land. For all these ancient rocks we may find a counterpart in 

 sediments now forming, and we may conclude with perfect confi- 

 dence that the ancient rocks were formed by the same agencies as 

 the modern accumulations. Every rock contains a more or less 

 legible record of its own history. 



Summary. — The brief survey of dynamical geology which we 

 have now taken has brought to light many facts of the highest sig- 

 nificance for the interpretation of the earth's history as recorded 



