684 



THE WONDERS OF GEOLOGY. Lect. VII. 



slates and calcareous sandstones (b, b) of the Devonian or 

 Old Red system. The section (Jig. 2), from north to south, 

 shows the carboniferous strata (a), flanked on the north 

 side only by the Devonian slaty rocks (b\ the granite of 

 Dartmoor (c) having been protruded on the southern edge ; 

 while the Devonian system re-appears in the southern part 

 of the county, terminated by a band of micaceo-chloritic 

 schists, which are parallel to the great disturbing axis of 

 Cornwall and Devon, and are probably altered or meta- 

 morphosed sedimentary deposits.* 



17. Trap Rocks and Dikes of the Carboniferous 

 System. — The coal and its associated strata everywhere 

 exhibit proofs of the violent subterraneous movements 

 they have undergone since their original deposition : and 

 but few coal-fields are free from extensive faults and dis- 

 locations, by which the beds have been broken up and 

 thrown into different levels and positions. The entire 

 group is also often traversed by veins and dikes of intruded 

 volcanic rocks ; generally consisting of the hard, dark green, 

 fine-grained stone, called trap. 



In Yorkshire there is a trap-dike of prodigious extent 

 and thickness, named the Whin-sill, which traverses the 

 coal-measures, triassic sandstone, and lias, and passes from 

 High Teesdale to the confines of the eastern coast ; a dis- 

 tance of upwards of sixty miles. 



In Derbyshire, a trap-rock, which is in many parts 

 amygdaloidal, and from being mottled with green and 

 yellow has received from the miners the name of toad-stone, 

 is interpolated between the beds of mountain limestone of 

 that country, under circumstances of considerable interest. 



These phenomena can nowhere be studied with more 

 advantage than in the neighbourhood of Matlock, which 



* Classification of the older stratified rocks of Devonshire and Corn- 

 wall, by the Rev. Professor Sedgwick and Sir R. I. Murchison. 

 Annals of Philosophy, No. 89. April 1839. 



