698 



GEOLOGY. 



exposed, tilted up on the sides by the obtrusion of subjacent masses. This basin-shape 

 is of immense utility, since it has brought all the layers of the formation near to the 



Mendip Hills 



1. Old red sandstone. 



2. Mountain limestone. 



SECTION OF THE BRISTOL COAL FIELD. 



3. Millstone girt. 5. New red sandstone. 



4. Coal measures. 



6. hias. 



7. Superior Oolite. 



surface, and thus placed them within reach ; for had they followed a uniformly inclined 

 course without interruption, the lower seams would have sunk to depths inaccessible to 

 the art of man. The preceding section very clearly points to the cause of the concave 

 form of the coal-beds. It shows also that the carboniferous deposit must have been laid 

 down before the elevation of the Mendip Hills, and that the horizontal strata overlying 

 the coal, belonging to the saliferous and oolitic systems, must have been formed after the 

 disturbance took place, since they exhibit no symptoms of participation in it. 



Besides the proof of disturbance afforded by the concave form of the coal strata, they 



have been splintered and broken 

 in various directions, by the same 

 tremendous power to which their 

 general contour is due, and hence 

 the slips, faults, and dykes which 

 the miner encounters in travers- 

 ing a single stratum. The sim- 

 plest form of a fault, or dyke, is 

 shown in the engraving, the 

 corresponding beds on each side 

 being slightly thrown out of their 

 level, the fault itself having no 

 Fault in Coai-fieid great breadth. In many cases, 



however, faults have a considerable width, one of 22 yards occurring in Montagu colliery 

 in the Newcastle coal-field ; and instead of the beds thus broken presenting only a trifling 

 change of level, the difference is often enormous. In the Newcastle coal-field a fault occurs 

 termed the " ninety fathom hitch/' the strata on the opposite sides deviating from a common 

 line to that extent. A fault at Sheriff-hall, in the vale of the Esk, in Mid Lothian, throws 

 the strata out of the line of stratification no less than 500 feet, and in that locality 120 

 ascertained dislocations occur. They abound also in the coal-fields of Fife and Clack- 

 mannan, causing divergences to the extent of from between 500 and 1200 feet. The 

 extraordinary energy of the dislocating power is sufficiently proclaimed by the fact of 

 massy beds being fractured, and the broken parts separated from each other, by such 

 intervals as these, either through the elevation of the one or the subsidence of the other, 

 for it is impossible to say which has happened. Faults are sometimes repeated several 

 times within a very short distance, each involving the displacement of strata. However 



