56 



OUTLINES OF GEOLOGY. 



132. Next to the coal, the most valuable rock of the coal 

 formation is the iron-stone, an argillaceous carbonate of iron, 

 having a lithoid structure, and which occasionally contains 

 carbonate of lime. This is wholly wanting in some coal 

 fields, as in those of Newcastle and Lancashire, but is abun- 

 dant in those of Liege, Staffordshire, and South Wales. 

 When it does occur in a coal field, and limestone either be- 

 longs to the formation, or can be obtained in the vicinity, it 

 is among the richest sources of wealth. 



133. The only metallic veins which exist in this group 

 appear to be of galena, which, in the field of Newcastle, is 

 traced from the carboniferous limestone, through the coal 

 measures, into the millstone grit. This galena is accompa- 

 nied by blende. Calc and bitter spar, sulphate of baryta and 

 quartz, also occur in veins. Iron pyrites is very abundant 

 in this formation, occurring in the coal sometimes in such 

 quantities as materially to injure its quality. Many coal 

 fields are traversed by great d}^kes, which divide them, as it 

 were into chambers, and which are accompanied by faults. 



134. The basins of the coal formation rest not only on 

 carboniferous limestone, but on the carboniferous sand-stone, 

 and on rocks of the submedial and inferior orders. On the 

 other hand, true coal is never found above the triasic group 

 in the formations of the supermedial and superior order. 



135. The carboniferous limestone is usually compact, but 

 occasionally granular or lamellar. It contains imbedded crys- 

 tals of calc spar, and sometimes passes into magnesian and ar- 

 gillaceous carbonates of lime. We have seen that it alternates 

 with the lower beds of the coal measures ; it also alternates 

 with the upper beds of the surrounding group. Beds of marl 

 frequently occupy its joints, and it contains caverns, some- 

 times of great extent. 



The colour of the limestone passes from white through all 

 the shades of gray, to black. It is also coloured and varie- 



