Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/39

 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 live-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 occasionally 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. Buddle.

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