62 Rev. A. Sedgwick on the Geological Relations and 



formed completely through the terrace) offers the best line of communication 

 between the sea coast and the part of the coal basin to the south of Durham. 

 From this part of the great field, coals might be conveyed to the sea by a series 

 of nearly dead levels not more than twelve or fifteen miles in length, on which 

 it would not, I believe, be necessary to use a single fixed engine. Milhons of 

 tons of coal are destined in future times to descend through this gap to the 

 neighbourhood of Stockton. Indeed before long, all the remote parts of the 

 coal basin will be intersected with rail-roads, which will rise over the escarp- 

 ment of the yellow limestone, and meet, like converging rays, at the nearest 

 sea -ports. 



§ 6. Faults affecting the Limestone and the Coal Strata, Trap Dykes, ^c. ^c. 



The unconformable position of the magnesian limestone being admitted, it 

 follows of course that many faults and dislocations which traverse the coal 

 strata will not affect the overlying formation. As, however, the causes pro- 

 ducing these dislocations are not confined to any one epoch, many faults are 

 common to both formations, or at least pass out of one into the other without 

 any visible interruption. Of this kind there are two examples on the coast of 

 Northumberland : the first, in Tynemouth Castie cliff, is not of great extent, 

 but is well exposed in a fine natural section (Plate VI. fig. 1.); the second, 

 which is much more remarkable, is seen at Cullercoats. The ninetT/ -fathom- 

 dyke there cuts through the cliff, and produces an enormous down-cast to the 

 north, by which the magnesian limestone is once more made visible, and 

 brought down to the level of the beach (Plate V. fig. 2.)*. 



Another and a very interesting class of faults which intersect the coal 

 strata, are marked by the presence of trap dykes. It has been assumed that, 

 in the country above described, these dykes are strictly subordinate to the 

 coal formation ; and from thence it has been inferred that they never pass up 

 into the beds of overlying limestone : the conclusion may generally be true, 

 but it is not borne out by any satisfactory evidence. Many dykes of trap 

 were probably injected among the coal strata at the time of their first disloca- 

 tion, before the existence of any part of the overlying formations. Such dykes 

 cannot possibly traverse any part of the magnesian limestone ; but we know 



* If the fault at Cullercoats be identical with that which is supposed to pass under the quarry 

 of Whitley, it will be difficult to reconcile the fact, stated in the text, with the section of Whitley 

 quarry, given in the Society's Transactions (First Series, vol. iv. Plate IV.). At all events the 

 yellow limestone is certainly dislocated by the great dyke at Cullercoats. No trap is there visible. 

 The word dyke is applied by the miners of the north of England indifTerently to all highly in. 

 clined faults, whether trap be present or not. An ignorance of this use of the word di/fce has led 

 to occasional mistakes. 



