to which Rock Masses may be subjected. 117 



valve to be perpendicular to the direction in which the one 

 bed tends to move along the other. The shell, in its dis- 

 torted form, will no longer be equilateral ; one half of each 

 shell will be crumpled into a smaller space, while the other 

 half will be extended into greater breadth ; so that if there 

 be longitudinal folds on the valve, those on the former half 

 will be pressed together, and those on the latter will be di- 

 lated into greater breadth. An exactly similar effect will 

 be produced on both shells ; but the compressed half of one 

 will be opposite to the dilated half of the other. 



Again, suppose the beds to be acted on by forces tending 

 to compress them equally in a direction parallel to their sur- 

 faces. The shell will then be compressed in the same direc- 

 tion, so that, generally, the ratio of the length to the breadth 

 of the shell will be altered, but without that twisting which 

 will characterise the distorted form in the former case. In 

 the case of this paragraph, the direction of compression will 

 coincide with what has been above termed a principal direc- 

 tion., and it will also be that of maximum normal pressure. In 

 the pi-evious case, the common surface of the two beds will 

 be the plane of maximum tangential action. 



If, then, in any stratified mass, we observe the organic re- 

 mains to be regularly distoi'ted, and twisted from their origi- 

 nal forms, as above described, we may conclude that the 

 planes of stratification have nearly coincided with those of 

 maximum tangential action ; but if, on the contrary, the dis- 

 tortion consists only in compression of the shells in a given 

 direction along the surface of the bed where they are found, 

 we may conclude that the direction of maximum normal pres- 

 sure has nearly coincided with this direction of compression, 

 and was consequently parallel to the planes of stratification. 

 The masses in which distorted remains have been found, are 

 generally those which have been much disturbed. The dis- 

 turbing forces are those to which the distortions are to be 

 referred ; and it may be remarked, that in such cases the 

 directions of maximum and minimum pressure at any point, 

 would probably lie in a plane perpendicular to the strike of 

 the elevated beds, and that, consecjuently, the planes of maxi- 

 mum tangential action, which bisect the angles between those 



