£14 Mr. Tate on Basaltic Rocks. 



Origin. — Little need be said as to its origin ; for it is now 

 generally admitted that basalt is an igneous rock, which had 

 been in a state of fusion ; for its composition is similar to 

 that of lava, and its effects on adjacent strata, as already 

 described, are such as would be produced by heated matter; 

 the stratified rocks are hardened and prismatised, and some 

 are crystalline; soft shales are converted into hard cherts, 

 Lydian stones and jaspers ; common limestones into granular 

 marbles, and coal into cinders or anthracite. Throughout 

 the whole range in Northumberland such effects are seen, 

 wherever the mass is of considerable thickness. No doubt 

 the structural characters of basalt are different from those of 

 lava ; but this may be accounted for, as it has been shewn by 

 the experiments of Gregory Watt, that the structure of rocks, 

 which had been in a molten state, depends on the rate of 

 cooling. Basalts cooling slowly under pressure, and most 

 probably at a time when the land was beneath an ocean of 

 considerable depth, would then acquire their crystalline 

 structure and prismatic form. 



How has the Whin Sill been intruded among the strata, 

 and at what period^. Was it erupted during the Mountain 

 Limestone era, and poured over the rocks below it, before the 

 other rocks above it were deposited ? or was it ejected subse- 

 quently to the deposition of the whole Formation, and forced 

 among the beds as a lateral dyke, between the surfaces of 

 stratification ? The former view has been advocated by W. 

 Hutton, and it is mainly based on the stratiform appearance 

 of the Sill, and on its supposed definite position among the 

 strata. It is however only locally, and for no great distance, 

 that the Sill simulates a stratum ; an extended survey of it 

 shews that it is very irregular in thickness, and that the 

 parallelism of the upper and under surfaces is maintained for 

 but a short space. Everywhere in Northumberland this 

 irregularity is seen. Indeed, as is apparent at Hatcheugh, 

 the mass is wedge-shaped. Sedgwick remarks in reference 

 to the basalt of Teesdale — " that the trap on the south side of 

 the valley descends among the strata in the form of a great 

 wedge, which diminishes in thickness from thirty or forty 

 fathoms to about twelve feet;"* and that " on the north side 

 of the river below Caldron Snout, we find the base of the 

 trap gradually sweeping over the broken ends of the stratified 

 rocks."f 



* Trans, of Camb. Phil. Soc. vol. i., p. 163. 

 t Ibid, p. 161. 



