216 Mr. Tate on Basaltic Rucks. 



activity. Ratcheugh and the neighbourhood clearly indicate 

 at least two distinct eruptions ; and, during the same period 

 there are evidences, in the Shilbortle Colliery, of two basaltic 

 overflows, apart from the Whin Sill, and probably proceeding 

 from the basaltic dyke in Trobe's dene. The vomitories of 

 the basalt are not recognizable ; along the whole range no 

 crateriform hollows or cones appear ; nor can we suppose that 

 the perpendicular dykes by which the county is traversed are 

 large enough, or so situated, as to have been outlets for the 

 molten rock forming the Sill. Doubtless it issued at different 

 points along the line, which are now hidden by superincum- 

 bent strata. Sedgwick thinks it perfectly certain that the 

 basaltic rocks of High Teesdale were not formed after the 

 dykes of the Coal field ; it therefore follows, he says, " that 

 they must have existed in their present form before the 

 deposition of the Magnesian Limestone."* Phillips arrives 

 at a similar conclusion, but from different evidence : " The 

 Whin Sill," he says, " is of date anterior to the east and west 

 lead veins of Tynedale, Teesdale, and the Pennine chain, for 

 it is divided by these veins of fissure. The Whin Sill is 

 then older than most parts of the saliferous system. "f It is, 

 therefore, I think, highly probable that subsequent to the 

 deposition of the Mountain Limestone Formation, and prior 

 to the Permian era, Northumberland, while beneath the sea, 

 was rent by volcanic forces acting in a winding line, having 

 a general direction of N.N.E. to S.S.W., but which in its 

 first and more northern part is to the south-east ; in its second 

 portion to the south; in its third and longest portion to the 

 south-west ; and in its fourth to west by south ; and that at 

 remitting intervals, and from different openings, molten 

 rock was poured over the surface or forced among the strata. 

 Relation of perpendicular Basaltic DyJces to the Whin 

 Sill. — Though the perpendicular basaltic dykes in North- 

 umberland, cutting through strata, have not been seen in 

 contact with the Whin Sill, yet they are of the same origin ; 

 for they differ little in mineral character from it ; and when 

 a dyke is wide, such as that of Lindisfarne or Hampeth, the 

 chemical and mechanical effects produced by it are similar to 

 those resulting from the Whin Sill. When dykes are 

 narrow, the rock is finer in the grain, and the influence on 

 strata is very slight. Some dykes, however, as that at 



* Trans. Camb. Phil. Soc, Vol. II., p. 189. 

 t Geology of Yorkshire, Part II., p. 84. 



