856 



THE WONDERS OF GEOLOGY. Lect. VIII. 



from the wall of the dike, is completely metamorphosed. 

 Those portions of the chalk which have been exposed to the 

 extreme influence of the trap, are now a dark brown crys- 

 talline rock, the crystals running in flakes, like those of 

 coarse primitive limestone ; in the next state the rock is of 

 a saccharine structure — then fine-grained and arenaceous ; 

 a compact variety with a porcellaneous aspect, and of a 

 bluish-grey colour, succeeds ; this gradually becomes of a 

 yellowish-white, and passes insensibly into unaltered chalk.* 

 The flints in the hardened chalk are either of a yellowish, 

 or deep -red colour, and the chalk itself is highly phospho- 

 rescent. The fossils are much indurated, but retain their 

 usual appearance. 



To the south of Fairhead, in the county of Antrim, 

 syenite traverses mica-schist and chalk, and fragments of 

 the latter are impacted in the erupted mass, being changed 

 into granular marble. f The geological structure of that 

 part of Ireland consists of — 1, The underlying rock, Mica- 

 schist ; 2, Coal-shale ; 3, Triassic strata ; 4, Chalk. J 



In this place it is necessary to remind the reader of 

 the examples of intruded basaltic rocks which have been 

 noticed in the former part of this lecture, when treating 

 of the palaeozoic formations ; viz. the trap of Dudley 

 (p. 774), of the Malverns (p. 761), Abberley Hills (p. 778), 

 &c. ; the toad-stones of Derbyshire (p. 684), and the 

 Whin-sill of Yorkshire. The latter is an enormous basaltic 

 dike, which traverses the island from the Tees to Robin 

 Hood's bay, and intersects all the strata from the lowermost 

 beds of the coal-measures to the oolite inclusive. 



27. Trap Dikes in the Isle of Sky. — In the Isle of 



* Dr. Berger on the Geological Features of the North-east of Ireland, 

 (!eol. Trans, vol. iii. p. 172. 



f The beautiful statuary marble of Carrara is oolitic limestone, 

 metamorphosed by the influence of contiguous igneous rocks. 



X Mr. Griffiths. 



