IMBEDDED BASALT. 



139 



period; it might be protruded, laterally, between them. That such 

 lateral protrusions have actually taken place in some instances, is 

 proved by Dr. MacCulloch's observations on the coast ol Scotland, 

 where trap may be seen forming beds between strata of sandstone, 

 then suddenly cutting through the upper strata and forming other 

 beds above. See Plate III. fig. 3. where strata of sandstone are in- 

 tersected, vertically, by a dyke of basalt, and laterally by nearly ho- 

 rizontal beds of the same basalt. Professor Sedgwick has bestowed 

 much labor in investigating the true position of the Great Whinstone 

 Sill, and its relations to the different strata in its vicinity, and has 

 given a very luminous and satisfactory description of the remarkable 

 phenomena which it presents, proving unanswerably the igneous ori- 

 gin of this rock. 



It would be doing great injustice to this valuable paper, to attempt 

 an abridgment of the detail of interesting facts and arguments which 

 it contains : 1 shall briefly recapitulate some of the observations. 

 The whin sill is not a regular bed interposed between the same strata 

 in different parts of its range, but it cuts through or overlies very dif- 

 ferent strata. It has had an extraordinary effect in converting beds 

 of shale, on which it lies, into a porous slag ; and where the whin sill 

 comes in contact whh limestone, the limestone is converted into a 

 dull white granular and crystalline mass. (Query Dolomite ?) 



This conversion takes place not only in the subjacent limestone, 

 but sometimes on the limestone which covers the whin sill, — a fact 

 deserving particular attention, as it indicates that the whin sill was 

 protruded between the beds of limestone, otherwise it could scarcely 

 have produced any chemical or mechanical change on the upper bed 

 of limestone. In some parts, beds of hmestone are seen bent up- 

 wards and imbedded in the whin sill. 



Indeed Professor Sedgwick thinks it probable, that the whin sill 

 was produced by a lateral injection of volcanic matter, in a state of 

 igneous fusion. 



The beds of trap or toadstone, imbedded in the mountain lime- 

 stone of Derbyshire, were supposed by Mr. Whitehurst to have been 

 protruded or driven, in a melted state, between the strata : this opin- 

 ion was founded chiefly on the supposed fact, that the metallic veins, 

 which cut through the limestone, 1. 2. 3. do not pass into the toad- 

 stone, (see Plate IV. fig. 5.) and were therefore supposed to have 

 been broken through when the latter beds were protruded. It has, 

 however, since been discovered, that the veins do often pass into the 

 toadstone, though they seldom bear ore in this rock ; hence the con- 

 clusion of Mr. Whitehurst was deprived of its main support. Sub- 

 sequently, Mr. Farey, in his survey of Derbyshire, misled by an at- 

 tachment to theory, described the beds of toadstone as regular strata, 

 preserving their thickness and continuity through the Peak of Derby- 

 shire. This is by no means the case ; the beds of loadstone are ex- 

 tremely variable in their thickness and order of succession, and the 



