DIAGENESIS : LITHIFICATION 753 



structure, such as a calcareous or other concretion, exists in the 

 rock, this will resist compression more than the enclosing mud, and 

 so the layers of the latter may assume an upward or downward 

 curving attitude arching over or under the concretion, or, if the 

 latter is large, end abruptly against it, sometimes with the occur- 

 rence of slickensided surfaces on the exterior of the concretion. 

 Stylolites also belong in this category of pressure structures. They 

 will be more fully discussed beyond. 



That rocks are ordinarily under great strain from lateral and 

 vertical pressure has been shown by the fact that when the pressure 

 is relieved, as in quarrying, expansion takes place, while upward 

 bucklings of the quarry floor are also frequently observed. (Niles- 

 24; 25 ; Johnston-20; Cramer-9; 10.) 



The further phenomena resulting from pressure will be more 

 fully discussed under syinphnittisui or dynamo-metamorphism. 



2. Cementation. This is accomplished by the deposition, in the 

 pores of the rock mass and between the particles, of a substance 

 which will bind them together. The material is brought in solution 

 by the percolating rain or ground water, and may be derived from 

 a distance, from immediately adjoining formations, or from the 

 formation in question itself. Thus the calcareous sands of the 

 dunes on Bermuda are cemented by the rain water which percolates 

 through them and which dissolves some of the lime only to redeposit 

 it elsewhere in the same formation. The oolite grains of Gran 

 Canaria in the Canary Islands are cemented by lime deposited by 

 the sea water which is instrumental in forming these oolite grains. 

 This, as in the similar cementation of organic lime accumulations, 

 on coral reefs, etc., is brought about by the separation from the 

 water of additional lime, through the decay of the organic matter 

 and the formation of ammonium carbonates in the warm waters, 

 this chemical reacting with the lime salts in solution in the sea 

 water. Pleistocenic gravels are often cemented by the lime derived 

 from a partial solution of the limestone pebbles which they contain 

 or which are found in an overlying gravel or sand. Examples of 

 this are not uncommon in limestone regions. The great Pleisto- 

 cenic Nagefluh deposit of the Salzburg region is an example of the 

 cementation of a deposit in this manner never buried under younger 

 formations. The pebbles and grains of the rock- are so firmly 

 cemented that the galleries and crypts, cut into the formation and 

 dating back to the third century, are still perfectly preserved. 

 Pleistocenic delta deposits exposed near Lewiston, New York, and 

 formed when the ice front rested near the Niagara escarpment, and 

 before Lake Iroquois came into existence, have become consolidated 



