STRATIFIED ROCKS. 



183 



cases the process is very slow, and therefore the newer rocks 

 are often, though not always, imperfectly consolidated. 



2. Stratified Rocks have been gradually deposited. 



By this we mean that they have not been formed at 

 once, as some of the older geologists imagined, but by the 

 regular operation of causes similar to those now accumu- 

 lating sediments. The slowness was sometimes extreme. 

 For example : a. We have strata in which the laminae are 

 as thin as paper, and yet each one represents recurring 

 conditions, as ebb and flow of tide, or flood and low water 

 of rivers. 1). In some cases we have a shell attached to 

 the inside of another shell (Fig. 93), in such wise that the 

 latter shell must have been dead before the former 

 attached itself. In such cases 

 a half or quarter inch thickness 

 of rock represents the whole 

 life of the second shell, c. We 

 have seen that some limestones 

 are made up of the accumulated 

 remains of successive genera- 

 tions of microscopic shells 

 (page 115). Every inch thick- 

 ness of such deposit must rep- 

 resent a long period of time. 

 And yet such deposits are often 

 hundreds or even thousands of 

 feet in thickness. These are, 

 however, extreme cases of slow- 

 ness. As a general rule, coarser 

 materials are deposited more rapidly than finer e. g., 

 sands than clays and limestone, but all by regular opera- 

 tion of causes ; and therefore, making due allowance for 

 the nature of the materials, thickness is a rough measure 

 of time. 



3. Stratified Rocks were originally horizontal at 

 the Bottom of the Water, This is a necessary conse- 



FIG. 93. Serpulce on interior of a 

 shell. 



