/yb ON" THE HOEIZONTAL MO YEMENIS OE EOCKS, ETC. 



and, consequently, that the lateral compression has been very far 

 more than the curving taken alone would seem to indicate ; and 

 this especially applies to large folds. 



Again, as to the extent of the lateral compression of strata in 

 cases where it is not associated with any contortion, but is revealed 

 by the deformation of the contained fossils *, I would remark that 

 for the deformation of a fossil organism to furnish a measure of the 

 amount of thickening which the deposit containing it has undergone, 

 the organism must at the time it was subjected to the strain have 

 been as plastic as the deposit, a condition which, perhaps, will but 

 seldom have been fulfilled in the case of organisms durable enough 

 to insure their good preservation in the fossil state. 



That fossils do resist the deforming influence exerted by the thick- 

 ening of the deposit containing them is evidenced by the well-known 

 facts that thicker and harder shells are not found deformed where 

 thinner shells, Algse and Trilobites, associated with them in the same 

 formation, have suffered deformation t, and that sometimes particular 

 organisms are found less distorted in beds of one kind than in beds 

 of another at the same spot J. 



Again, where no contortion of the strata, or deformation of fossils, 

 affords evidence of lateral compression, we frequently have an indica- 

 tion of its occurrence in the simultaneous thinning of a series of 

 different strata in the same direction, and that whether the conver- 

 gence of the surfaces separating the strata is only slight or very 

 great, as in the well-known fan-shaped structure often displayed in 

 mountains. 



For I submit that for a series of superimposed deposits to be 

 originally laid down all having their thickness increasing in the same 

 direction, would seem to involve that during the whole period of 

 their deposition the position of the shore-line continued nearlj^ the 

 same ; and that, as this seems untenable, we must suppose that, 

 generally, deposits thus related have been thickened up at one 

 place, or thinned out at another, since their deposition. And the 

 only agent we know of, adequate to produce this effect on a large 

 scale, is lateral compression. 



The thinner as well as the thicker parts of the deposits will 

 generally, it is manifest, have been thickened in the process. 



These conclusions appear to me to have some interest and import- 

 ance, because the thickness of deposits is very generally regarded as 

 furnishing a clue to the length of time which was taken to form 

 them. If they are sound, we must, I think, conclude that most 

 indurated and disturbed strata have suffered considerable thickening 

 by lateral compression since their deposition. 



* See Dana's 'Geology,' 3rd edition, p. 98. 



t " Eeport on Cleavage and Foliation," John Phillips. Brit. Assoc. Eep. 

 1856, p. 386. 



X Sharpe, " On Slaty Cleavage," Quart. Journ. Geol. Soc. vol. iii. 1847, p. 77. 



