PART I. CHAPTER V. 



67 



Experiments to illustrate curved Strata. 



equal space, has been well described by Sir James Hall.* It 

 occurs near St. Abb's Head, on the east coast of Scotland, where 

 the rocks consist principally of a bluish slate, having frequently 

 a ripple-marked surface. The undulations of the beds reach 

 from the top to the bottom of the cliffs from 200 to 300 feet in 

 height, and there are sixteen distinct bendings in the course of 

 about six miles, the curvatures being alternately concave and 

 convex upwards. 



An experiment was made by Sir James Hall, with a view of 

 illustrating the manner in which such strata, assuming them to 

 have been originally horizontal, may have been forced into their 

 present position. A set of layers of clay were placed under a 

 weight, and their opposite ends pressed towards each other with 

 such force as to cause them to approach more nearly together. 

 On the removal of the weight, the layers of clay were found to 

 be curved and folded, so as to bear a miniature resemblance to 

 the strata in the cliffs. We must, however, bear in mind, that 

 in the natural section or sea-cliff we only see the foldings imper- 

 fectly, one part being invisible beneath the sea, and the other, or 

 upper portion, being supposed to have been carried away by de- 

 nudation, or that action of water which will be explained in the 

 next chapter. The dark lines in the accompanying plan (Fig. 



)f 



60.), represent what is actually seen of the strata in part of 

 Fig. 60. 



the 



line of cliff alluded to ; the fainter lines, that portion which is 

 concealed beneath the sea level, as also that which is supposed 

 to have once existed above the present surface. 



We may still more easily illustrate the effects which a lateral 

 thrust might produce on flexible strata, by placing several pieces 

 of differently coloured cloths upon a table, and when they are 

 spread out horizontally, cover them with a book. Then apply 



* Edin. Trans, vol. vii. pi. 3. 



