236 Dr Hibbert on the Limestone of Burdiehouse, 



Fractures among the coal strata, more or less inclined to pa- 

 rallelism, having been thus induced, a subsequent effect produced 

 by the struggle of disjointed portions of the earth's surface to 

 conform themselves to the diminished capacity of the internal nu- 

 cleus of the globe, would be manifested in the edges of these frac- 

 tures being pressed forcibly against each other, by which some of 

 the strata bounding the edge of a line of fracture would give way 

 to this mutual compression, and would consequently be thrust up- 

 wards in the form of mountain ridges. 



Accordingly, these mountain ridges observe lines of direction 

 which are often parallel, or nearly so, to lines of fracture. 



It has been thus shewn, that the Mid-Lothian coal-district ex- 

 hibits lines of fracture nearly parallel to each other ; — that causes in 

 operation had a tendency to force these broken surfaces into a 

 nearer contact with the centre of the earth, in order to adapt 

 themselves to the diminished capacity of the nucleus of the globe ; 

 that these causes had induced the edges of these fractures to ap- 

 proximate to each other ; and that in the mutual force and struggle 

 thus used, such edges of fractures as during the struggle had 

 given way, were thrust up into mountain ridges. 



It must also be evident, that in the case of any newer con- 

 tracted state of the nucleus of the globe, inviting a fresh struggle 

 of its broken crust to adapt itself to an internal surface of less ca- 

 pacity, old lines of fracture would of course be the lines of least 

 resistance. And hence, broken up and tilted masses would con- 

 tinue to be thrust up in the same line of direction. 



These efforts of a subsequent date appear to have been gra- 

 dual, and even oscillatory, as will be explained hereafter. Con- 

 sequently, few or none of the fractured systems of strata which I 

 have described, shew at their lines of fault marks of sudden vio- 

 lence ; — a circumstance which had been previously observed by 

 Williams, in the instance of the great fault which divides the 

 Hawthornden and Burdiehouse systems. In short, every thing 

 indicates, that these later movements must have taken place by 

 very slow, prolonged, and tranquil processes. 



