INTERSTRATIFIED BASALT. 



145 



of the alternation of the beds of basaltic amygdaloid, with beds of 

 h'mestone in other situations. But in both instances, we must admit 

 that the beds were formed under the ocean, before the present islands 

 and continents had emerged from the watery abyss. With respect 

 to Etna, the alternation of lava and limestone affords decisive evi- 

 dence, that this mountain was upheaved from the ocean, though its 

 height may have been greatly augmented by eruptions of lava, since 

 the period of its first elevation. Before concluding the account of 

 interstratified basalt, it may be proper to mention, that Mr. William 

 Hutton, in a paper lately read to the Geological Society of London, 

 maintains, that the great bed of basalt in Northumberland, called the 

 whinstone sill, was deposited over the limestone beds on which it 

 rests, and not protruded laterally between them : though he admits, 

 with Professor Sedgwick, that the basaltic beds in Teesdale were 

 protruded in the manner before described. In some cases, it may 

 be extremely difficult to determine whether a bed of basalt has 

 flowed like laya, or been protruded laterally, because two strata of 

 hard limestone, for example, are often separated by seams of soft 

 clay, which would dispose the beds to yield to a lateral pressure in 

 the direction of the strata, and the injected basalt would take the 

 form of a regular stratum. 



Mr. Hutton admits, that in some instances, the limestone over the 

 basalt had suffered the same effects of igneous action as the lime- 

 stone below it. These instances, I think, afford satisfactory evidence, 

 that the basalt was protruded between the beds of limestone. 



The disturbances and contortions of some of the lower beds of 

 transition limestone, in the vicinity of trap rocks, were mentioned in 

 Chap. VII. In such instances, though frequently no visible connec- 

 tion between the rocks of trap and limestone can be traced on the 

 surface, there can be little doubt that such connection exists. The 

 singularly bent limestone beds at Wren's Nest Hill near Dudley are 

 at a considerable distance from the nearest basaltic hill ; but I ob- 

 served in the town of Dudley, where a well was sinking, that the 

 stone thrown out was granular basalt, intermixed with calcareous spar. 



Some species of trap rocks, and particularly the softer kinds of 

 basalt, decompose rapidly, and form productive soils and marie. I 

 am inclined to believe, that some of the most fertile soils in England 

 were formed, by an intermixture with decomposed basaltic rocks. 

 What has been called basaltic tufa, is a volcanic substance, and will 

 be described among volcanic products. Some of the trap rocks, 

 particularly the porphyriiic traps, are metalliferous ; but it is rarely 

 the case with any of the British trap rocks, and it has before been 

 stated, that the veins of lead ore in Derbyshire, are either cut off 

 by beds of basalt, or generally cease to yield ore, when passing 

 through basalt. 



Having described the principal phenomena attending trap rocks, 

 whether occurring in dykes, in unconformable masses, or interslrati- 



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