188 



STRATIFIED OR SEDIMENTARY ROCKS. 



that an original ideal sphere has been changed into an ellipsoid, whose 

 greatest and shortest diameters are to each other, in some cases, as 2 : 1, 

 in others as 3 : 1, 4 : 1, 6 or 7 : 1, 9:1, and in some even 11 : 1. The 

 average in well-cleaved slates, according to Sorby, is about 6 : 1. Now, 

 since this ratio is the result partly of compression and partly of exten- 

 sion, it is evident that either the compression alone or the extension 

 alone would be the square roots of these ratios. Therefore, we may 

 assume the average compression as 2-J : 1, and the average extension 

 as 1 : 2|, 



It is impossible to overestimate the geological importance of these 

 facts. Whole mountains of strata, whole regions of the earth's crust, 

 are cleaved to great and unknown depths, showing that the crust has 

 been subjected to an almost inconceivable force, squeezing it together 

 in an horizontal direction and swelling it upward. This upward swell- 

 ing, or thickening of the strata by lateral squeezing, is a probable cause 

 of gradual elevation of the earth's crust, which has not been noticed by 

 geologists. We will speak again of this important subject in our dis- 

 cussion of mountain-formation. 



There are reasons for believing that the squeezing did not take 

 place, and the structure was not formed, while the strata were in their 

 original condition of plastic sediment, but after they had been consoli- 

 dated into rock and the contained fossils had been completely petrified, 

 otherwise the shells must have been broken by the pressure. Yet, on 

 the other hand, some degree of plasticity seems absolutely necessary to 

 account for so great a compression in one direction and extension in 

 another without disintegration of the mass. It seems most probable 

 that at the time the structure was produced these rocks were deeply 

 buried beneath other rocks and in a somewhat plastic state, through 

 the influence of heat in the presence of water. Afterward, they were 

 exposed by erosion. 



Nodular or Concretionary Structure. 



In many stratified rocks are found nodules of various forms scattered 

 through the mass or in layers parallel to the planes of stratification. 

 Like slaty cleavage, this structure is the result of internal changes sub- 



Fig. 167. 



Fig. 



sequent to the sedimentation ; for the planes of stratification often pass 

 directly through the nodules (Figs. 167 and 168). The flint nodules of 



