CHANGES IN STRATA 221 



Somewhat similar changes in the strata may be occasioned by 

 the steady lowering of a land surface through denudation. This 

 diminishes the velocity of the streams, which, in its turn, changes 

 the character of the materials which the rivers bring to the sea. 



We have no trustworthy means of judging how long a time was 

 required for the formation of any given stratum or series of strata, 

 but it is clear that different kinds of beds accumulate at very 

 different rates. The coarser materials, like conglomerates and 

 sandstones, were piled up much more rapidly than the shales and 

 limestones ; so that equal thicknesses of different kinds of strata 

 imply great differences in the time required to form them. Com- 

 paring like strata with like, we may say that the thickness of a 

 group of rocks is a rough measure of the time involved in their 

 formation, and that very thick masses imply a very long lapse of 

 time, but we cannot infer the number of years or centuries or 

 millennia required. 



Geological chronology can be relative only. Such a relative 

 chronology is given in the section that we have examined by the 

 order of succession of the beds. Obviously the lowest stratum is 

 the oldest and the one at the top the newest. This may be put as 

 a general principle, that, unless strata have lost their original posi- 

 tion through disturbance or dislocation, their order of superposi- 

 tion is their order of relative age. It is for this reason that in 

 geological sections the strata are numbered and read from below 

 upward. 



Change in the character of the strata takes place not only verti- 

 cally, but also horizontally^ since no stratum is universal, even for 

 a single continent. Our study of the processes of sedimentation 

 which are now at work, showed us that the character of the bottom 

 in the ocean or in lakes is subject to frequent changes, varying 

 with the depth of water and other factors. The same is true of 

 the ancient sea and lake bottoms, now represented by the strati- 

 fied rocks of the land. Strata may persist with great evenness 

 and uniform thickness over vast areas, and in such cases the bed- 

 ding planes remain sensibly parallel. But sooner or later, the beds, 

 whenever they can be traced far enough, are found to thin out to 



