176 DESCRIPTION OF THE STRATA. 



coal strata, the quality of the coal is changed ; not only where it 

 comes in contact with the dyke, but for several yards on both sides 

 of it ; appearing charred or burnt. This effect has been produced by 

 our dyke, where it traverses the coal at Cockfield ; the coal being 

 reduced to a cinder, and the sulphur sublimed from the pyrites. 

 Were the junction of the dyke with the coal seams at Maybecks, 

 and other parts of our district, exposed, we might expect to witness 

 the same phenomena. It would be desirable to know what effect the 

 dyke has had on our alum shale ; but we have not found any spot 

 where the basalt and the shale are seen to come in contact. We 

 have observed, however, that in its passage through the sandstone 

 and the alluvium, the pai'ting between the dyke and the strata which 

 it traverses is commonly very distinct, so that there is no diflSculty in 

 detaching the basalt ; and when it is removed, the sides of the vein 

 appear lined with several coatings of a clayey substance, hard and 

 whitish. In some places, this crust is smooth, like plaster or stucco ; 

 in others, we find the surface adorned with raised lines, curiously 

 reticulated and ramified, in a manner analogous to the dendritical 

 impressions, so frequently observed in the partings of sandstone and 

 other rocks. This curious coating often looks as if it had been 

 baked ; while in some spots there are perpendicular markings, which 

 seem to have been made by the motion and friction of the whole vein, 

 xipwards or downwards, rubbing against its sides. The substance is 

 not a pure clay, but contains a mixture of calcareous and siliceous 

 matter; numerous small fragments and grains of various kinds of 

 stone being imbedded in the clayey cement. 



