54 LYELL'S ELEMENTS OF GEOLOGY. 



Consolidation of Strata by Pressure and Heat. 



took place, and the carbonates of lime and magnesia separated 

 from the more impure and mixed matter forming the still unconso- 

 lidated parts of the stratum. Crystallization, beginning at the 

 centre, must have gone on forming concentric coats around the 

 original nucleus, without interfering with the laminated structure 

 of The rock. As to the radiations from a centre, it is a phenome- 

 non which, however singular, is common in spherical concretions 

 of various mineral ingredients. 



When the particles of rocks have been thus re-arranged by 

 chemical forces, it is sometimes difficult or impossible to ascer- 

 tain whether certain lines of division are due to original deposi- 

 tion or to the subsequent aggregation of similar particles. 



p. _ Thus suppose three strata of grit, 



A, B, C, are charged unequally with 

 f calcareous matter, and that B is the 



III 



Bi- 





most calcareous. If consolidation 

 takes place in B, the concretionary 

 action may spread upwards into a 

 part of A, where the carbonate of lime is more abundant than in 

 the rest ; so that a mass, d, e, jf, forming a portion of the supe- 

 rior stratum, becomes united with B into one solid mass of stone. 

 The original line of division df, e, being thus effaced, the line, 

 d,f, would generally be considered as the surface of the bed B, 

 though not strictly a true plane of stratification. 



Pressure and heat. When sand and mud sink to the bottom 

 of a deep sea, the particles are not pressed down by the enormous 

 weight of the incumbent ocean ; for the water, which becomes 

 mingled with the sand and mud, resists pressure with a force 

 equal to that of. the column of fluid above. The same happens 

 in regard to organic remains which are filled with water under 

 great pressure as they sink, otherwise they would be immediately 

 crushed to pieces and flattened. Nevertheless, if the materials 

 of a stratum remain in a yielding state, and do not set or solidify, 

 they will be gradually squeezed down by the weight of other 

 materials successively heaped upon them, just as soft clay or 

 loose sand on which a house is built may give way. By such 

 downward pressure particles of clay, sand, and marl may become 

 packed into a smaller space, and be made to cohere together 

 permanently. 



Analogous effects of condensation may arise when the solid 

 parts of the earth's crust are forced in various directions by those 

 mechanical movements afterwards to be described, by which 

 strata have been bent, broken, and raised above the level of the 

 sea. Rocks of more yielding , materials must often have been 



