692 GEOLOGY. 



alternating beds of a different composition, layers of chert, or nodules of a hard siliceous 

 nature, corresponding to those of flint, which so commonly occur in the chalk cliffs. 

 The limestone, however, sufficiently predominates to become characteristic. It yields the 

 lime of agriculture upon being burnt, and that used for mortar, the action of heat 

 changing its colour to a pure white, by destroying the animal and vegetable remains 

 which impart to the rock its various shades of light and dark grey. The black variety 

 also, frequently used for chimney-pieces, has its colour, probably arising from bitumen, 

 completely expelled by burning. The limestone exhibits a well-defined stratification, a 

 compact structure, and forms marbles of great beauty upon being polished. The 

 romantic rocks of Cheddar flanking the chain of the Mendip Hills in Somersetshire, the 

 heights on each side of the Avon at Clifton, the picturesque cliffs on the banks of the 

 Wye, the bold tors of Derbyshire, and the high mural precipices laved by the Meuse 

 below Namur, are examples of the mountain limestone, which, for scenic effect, upon a 

 limited scale, 



" So wondrous wild, the whole might seem 

 The scenery of a fairy dream," 



stands at the head of the rock -formations of the globe. This part of the carboniferous 

 system presents numerous fissures and caverns, almost all the grand examples of these 

 structures being found in it. They have either arisen from irregular beds of sand 

 and clay deposited with the limestone, and subsequently washed away by the agency 

 of water, or from small fissures in the rock, formed by the elevating forces that have 

 acted upon it, which, in the course of ages, the constant percolation of water has en- 

 larged into the chasms that now yawn in the rocky fabric. The stream that issues 

 from the mouth of the cavern at Castleton the general attendant of such excavations 

 points to one part of the machinery that has operated in their construction ; and the 

 disturbance of the strata by the protrusion of igneous rocks is clearly indicated in other 

 localities. We have previously given a view of the insulated dome-shaped hill at 



Crich in Derbyshire, which 

 supplies an example of the 

 arched arrangement of strata ; 

 and the cut represents a sec- 

 tion of this protuberance, 

 a remarkable object in the 

 district, showing, in the dark shaded centre, a protrusion of compact trap, reached 

 upon striking a shaft in search of lead ore, which explains the curved form of the 

 overlying limestone mass. The hill supplies many instances of cracks and fissures, filled 

 with metallic ores and spars, formed by an intense heat acting under immense pressure. 



The mountain limestone is remarkably rich in metallic products ; and hence it has 

 been sometimes called the metalliferous limestone. It is the principal depository of 

 the lead found in Great Britain, the chief mines of which are in the northern coun- 

 ties of England, in Derbyshire, in North Wales, and in Devonshire on the borders of 

 Cornwall. These mines, with the Scotch and Irish, yielded the following produce in 

 the year 1828: 



Section of the Crich-hill. 



North of England . , - - , - 26,700 tons. 

 Derbyshire and Shropshire - - 4,800 

 Devonshire and Cornwall - - 2,000 



Flintshire and Denbighshire - 1 2,000 tons. 



Scotland - - 1,000 



Ireland, and Isle of Man - - 500 



Total 47,000 tons. 



The Derbyshire lead-mines, among the most ancient in the kingdom, are in the 

 north-west part of the county, extending as far south as the neighbourhood of Matlock, 



