554 MANUAL OF NATURAL HISTORY. 



Iron-ore or clay-ironstone, from which the valu- 

 able product iron is extracted, is also found in this 

 formation, which thus produces the mineral, and the 

 requisite fuel for smelting it. In 1846, two mil- 

 lions two hundred and fourteen thousand tons of 

 iron, of the value of eight millions eight hundred 

 and fifty-six thousand pounds, were produced in 

 England. 



Igneous Rocks are frequent in the Carboniferous 

 group, sometimes occurring as overlying stratiform 

 masses, alternating with sedimentary deposits, and 

 frequently as dykes, penetrating through the strata. 

 They principally consist of greenstone and basalt 

 or whinstone, in the north of England, the mottled 

 toadstone of Derbyshire, and the basaltic masses of 

 South Staffordshire. 



The Mountain Limestone, which naturally be- 

 longs to this group, is composed of thick-bedded, 

 grayish limestones, and shales, with layers and no- 

 dules of chert, and ores of lead, zinc, and copper, 

 with baryta, and fluor-spar. It is a marine deposit, 

 abounding in the crinoid forms of Echinoderms, and 

 the shells of cephalopodous and brachiopodous mol- 

 lusks. It is extensively developed in the northern 

 and western parts of England, rising in picturesque 

 peaks and hills, as in Derbyshire, Cumberland, and 

 Westmoreland. The mountain limestone of Eng- 

 land is rich in lead, producing annually 30,000 tons, 

 equivalent to about five hundred thousand pounds 

 sterling ; it occurs chiefly in the form of sulphuret 

 of lead, or galena ; manganese is also found in this 



