PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. % 



z K 



Fig. 7. 



^ 



the front part of the shell, its thickest part is pressed over and par- 

 tially conceals its upper portion : the unnatural curves assumed by 

 the ribs help to show the direction in which it has given way. Be- 

 sides this forward pressure, there is evidence, as in the former 

 cases, that the whole has been compressed together in the direc- 

 tion perpendicular to the planes of cleavage and lengthened in 

 the direction of the dip of the cleavage, by which the proportion of 

 the two sides of the shell, and also of each fold and rib, has been 

 altered. Another specimen of a cast of the same species, which has 

 suffered still more alteration, is copied in fig. 7 : the larger part of 

 the shell has been squeezed under and concealed, while the re- 

 mainder is so much expanded that the impressions of the hinge-plates 

 are nearly double their usual length, the expansion being as usual in 

 the direction of the dip of the cleavage. 



To understand the effect that must be produced upon an arched 

 body by a contraction of the mass in which it is imbedded in one 

 direction and its expansion in another, we must recollect that the 

 parts of the body lying in different planes will be differently affected 

 by the two forces ; for instance, in fig. 8, let C A D be the section 



Fig. 8. Fig. 9. 



x/ xr 



of an arched shell lying in a mass of rock intersected by the cleavage 

 planes whose section is seen in YY ; if the mass is expanded in the 

 direction of Y Y (the dip of the cleavage) and contracted in the 

 contrary direction X X, drawing with it in each case the impression 

 of the shell, the line CAD of fig. 8 will be changed by one of its 



