Mr. Tate on Basaltic Rocks. 215 



That the Sill occupies a definite place in the Mountain 

 Limestone Formation is founded on what appears in the mining 

 districts of Tynedale and Teesdale ; and, indeed, so far has 

 the notion been carried, as to have made the basalt to be the 

 line of division between the Yordale series and the Scar lime- 

 stone. But in Northumberland it has no such exact position. 

 It always appears in connection with the calcareous division 

 of the Mountain Limestone Formation. It never enters either 

 the Coal Measures or the Millstone Grit; and though, in 

 some places Boulder Clay lies immediately above it, yet that 

 clay has not in any way been disturbed by it. Its position, 

 however, in the calcareous division, alters very much in the 

 course of its range ; in one part its relative vertical position 

 is above 1,000 feet lower than in other parts ; and this con- 

 clusion is drawn from a careful consideration not only of 

 natural sections, where the chief limestone beds are seen, but 

 also of their organic contents. The course of two beds of 

 this limestone — widely apart from each other — are by these 

 means pretty well ascertained : the one, the Ebb's Nook lime- 

 stone, which is about 330 feet from the top of the series ; 

 the other, the Hobberlaw limestone, which is about 1,450 feet 

 from the top. Now, in one part of the range, the Sill is above 

 the Ebb's Nook limestone, and in another part its position is 

 near to the Hobberlaw limestone ; it is therefore evident that 

 the Sill cuts through, here and there, the series of strata, and 

 that it has been erupted subsequently to the deposition of, 

 certainly, nearly the whole, if not the whole, of the Mountain 

 Limestone Formation; where, therefore, the basalt appears in- 

 tercalated with the strata it has been forced into themasalateral 

 dyke along the planes of stratification. The marked effect of 

 the basalt on the beds above it, helps to confirm this conclu- 

 sion ; for these beds, in some cases, are more altered than the 

 beds below it. The mechanical disturbances produced by the 

 basaltic eruption furnish additional evidence ; the tilted and 

 reversed strata at Little Mill, which are about 770 feet from 

 the top of the series, prove that the basalt was erupted there 

 after — and probably long after — these beds had been deposited; 

 and the great masses of strata displaced by the basalt at the 

 Fame Islands, and especially at Cullernose, tell of the great 

 power of the eruption. 



Whatever may be the age of the basalt, the whole had not 

 been erupted at one time. Volcanic action extended over a 

 long period, and there had been times of repose as well as of 



