§ 17. TRAP ROCKS AND DIKES OF DERBYSHIRE. 685 



is a region of carboniferous limestone, broken up and tra- 

 versed by volcanic rocks. 



The basalt or trap of Derbyshire was evidently erupted 

 in a state of fusion, from some very deep-seated source 

 of intense heat, and intruded between the limestone beds 

 in three principal currents, which now appear as alter- 

 nations of trap-rock with sedimentary strata : but which 

 there is every reason to believe sprung from one common 

 source, and are lateral protrusions from the great mass of 

 igneous matter. The toa cist one is a hard rock, con- 

 sisting of small nodules of white and yellow calcareous spar 

 and green earth, imbedded in a dark greenish paste of 

 basalt. Sometimes the nodules are decomposed, and the 

 stone is then vesicular or cellular, resembling porous lava. 

 Three distinct beds of this ancient lava-current are inter- 

 polated in the mountain limestone of this district ; the 

 thickness of each varies from sixty to eighty feet. In some 

 instances, dikes of toadstone traverse the metalliferous veins, 

 and a manifest alteration is then observable in the nature 

 of the latter.* 



In some places the elevatory movements have torn the 

 rocks asunder ; in others the strata have partially resisted 

 the expansive effect of the erupted lava, and are now in 

 a dome-shaped or arched position, and more or less bent 

 and folded. 



Crich Hill, near Matlock, affords a highly interesting 

 illustration of this effect of igneous action. It is a dome- 

 shaped hill of mountain limestone, 800 feet high, consisting 

 of arched strata, enfolding a central mass of trap. This 

 dome of limestone has been forced up through the once 



* In the cavern at the base of the High Tor in Matlock Dale, a 

 bed of toadstone is seen on the floor, beneath the limestone strata of 

 which the cliff is composed, and may be traced across the river to the 

 opposite escarpment of Masson's Hill, where it is exposed on the 

 road-side. See Medals of Creation, vol. ii. p. 945. 



