FURTHER NOTES ON TIDESWELL DALE QUAKK1ES. 



Figs. 9 and 10. 



unless, indeed, this expansion be resisted by another body pressing 

 upon it; but sideways it is otherwise, for as soon as the film begins to 

 form, its expanded particles, excepting those 

 near the edges, are squeezed together laterally. 

 Nevertheless, the cohesion which the film 

 has with the unaltered material is able at 

 first to overcome this state of lateral stress ; 

 but as the film increases in thickness, so is 

 the number of its altered particles multiplied ; 

 and hence also this lateral stress which is 

 exerted by the particles — the cohesion re- 

 maining a constant quantity. Hence a time must come when this 

 cohesion of the film with the unaltered material is no longer able 

 to hold in check this stress in all such places where the film has an 

 opportunity of responding to the expansion of the particles, a 

 rupture being the result (as b b. Fig. 10). 



We must now apply this, under certain modifications, to our 

 spheroids. Turn to Fig. 5 again, and observe that the " shells " 

 are not continuous all round the nucleus, but overlap, somewhat 

 like the petals of a garden rose. This almost explains itself : — 

 A film forms, and at length attains 

 to such a thickness that the resultant 

 state of lateral stress must be relieved 

 in one of two ways — the envelope 

 of film as a whole may be thrown 

 off from the nucleus, or it may be 

 sheared obliquely where it is weakest 

 or thinnest — the one part sliding over the other laterally (Fig. 1 1 , a a). 

 The former cannot take place on account, partly of the closely 

 packed condition of the rock offering resistance to all expansion 

 away from the nuclei, and partly of the unequal thickness of the 

 film itself. When a film has thus separated, a new one is formed 

 beneath it, to separate in due course from its parent stone in like 

 manner, and the process may go on, till at length no unaltered 

 material is left. 



Fig. 



