682 



THE WONDERS OF GEOLOGY. Lect. VII. 



principal metalliferous districts of Derbyshire, are so fully 

 described in various works, that a brief notice of the manner 

 in which the mineral and metallic productions occur, will 

 suffice. 



The chasms in the limestone exposed in some of the 

 quarries, are often more or less incrusted with minerals 

 and spars, and exemplify the mode in which the rich metallic 

 veins of lead are distributed in the interior of the moun- 

 tains.* A fissure which I observed in a quarry in Crich 

 Hill, will serve as an illustration. A layer of the blue sul- 

 phuretof lead, or galena, was spread over the surface of the 

 limestone that formed the walls of the fissure, and upon this 

 was deposited a thick stratum of white sulphate of barytes ; 

 on the latter was a coating of fluor spar of a light blue 

 colour, crystallized in cubes on the surface, and the cavity 

 left in the rock was lined with cubic crystals of fluate of 

 lime.f 



16. Carboniferous System of Devonshire. — The 

 rapid sketch I have given of the principal features of the 

 three groups of strata composing the Carboniferous system, 

 affords a general idea of the prevailing characters of this 

 important formation. There is, however, an extensive 

 series of rocks, which belong to this epoch, but occur 

 under conditions that rendered their relations somewhat 

 obscure, and occasioned them to be classed with the more 

 ancient beds, until the investigations of Professor Sedgwick 

 and Sir E. I. Murchison demonstrated their true position 

 in the chronological arrangement of the British strata. This 

 group consists of shales and slaty coal, provincially called 

 culm, constituting a trough of carboniferous deposits super- 

 imposed on Devonian sandstone, but much dislocate! and 

 altered in character by intrusions of granite and other 



* See " Excursion to Crich Hill :" Medals of Creation, vol. ii. 

 P. 951. 



f Ibid. p. 955. 



