Northumberland and Durham. 29 



observed in other parts of the kingdom,; and render evident the 

 operation of a most powerful agent employed in tearing up the 

 surface, and in dispersing the fragments of the ruin. 



In the coal measures near the edges of those dykes rounded 

 pebbles of sandstone and fragments of coal cemented together by 

 sand are sometimes met with ; as in Lawson main, Sheriff hill, and 

 Montagu Main collieries. 



Galena has been found in a dyke in Willington colliery, and a 

 small string of the same ore has been observed in the main dyke at 

 Whitley. A salt spring issues from a slip in Birtley colliery. 



The dykes are an endless source of difficulty and expense to the 

 coal owner, throwing the seams out of their levels, and filling the 

 mines with water and fire damp. At the same time they are not 

 without their use ; when veins are filled, as is often the case, with 

 stifF clay, numerous springs are damned up and brought to the 

 surface ; and by means of downcast dykes valuable beds of coal are 

 preserved, which would otherwise have cropped out and been lost 

 altogether. Thus the high-main, the five-quarter, and the seven- 

 quarter coal seams would not now have existed in the country to 

 the north of the main dyke but for the general depression of the 

 beds occasioned by that chasm. 



The other irregularities observed in the coal measures are the 

 following : 



1. Large wedge-shaped portions of the strata that are occasion- 

 ally found to have sunk from their level. This occurrence was 

 noticed in Cockfield colliery by Mr. Dixon, and a section of it is 

 given in the history of Durham. A much more serious difficulty 

 of the same kind was surmounted within these few years in 

 Hebburn colliery by Mr. Buddie. 



2. Fissures that divide the strata, but do not alter their level, 



