236 Outlines of Geology. 



acquire a considerable degree of importance and interest as being 

 the principal seats of the useful metals, and as abounding in this 

 country in lead, copper, tin, and some other metallic bodies. 



The characters of the limestone of the lead measures are very 

 variable ; in some places it abounds in madreporites, encrinites, and 

 other fossils, and is hard enough to receive a polish; in other 

 places, from the prevalence of argillaceous matter, it acquires a 

 bluish colour, softer texture, and loses its organic remains. In this 

 great limestone district, the lead veins hitherto worked occupy a 

 space of about fifteen miles N. and S., and twenty W. and E., and 

 they run with little exception east and west. They present some 

 peculiarities which are worthy of particular remark, although per- 

 fectly inexplicable, and to which I shall afterwards more explicitly 

 allude. Among these, their varying dimensions, depending upon the 

 nature of the beds they traverse, are particularly curious. In pass- 

 ing through the argillaceous strata, they become narrow, and even 

 Waste away into almost imperceptible threads, which again re- 

 unite in the limestone, thicken, and often bulge out into prolific 

 bunches of ore ; a vein, not more than two or three feet wide in the 

 shale, and mixed with rocky matter, shall suddenly become pure 

 in the limestone, and widen to between seventy or eighty feet. 



Though these veins contain several varieties of lead ore, the 

 siilphuret or galena is their only important product. The best 

 ores yield about two- thirds their weight of pure lead, and often 

 contain no inconsiderable portion of silver. We learn from Mr. 

 Phillips, that between 1803 and 1810, the greatest quantity of 

 lead shipped in any one year at Newcastle, was 10,352 tons — the 

 least 3,910. The average shipment at Stockton is about 3,000 

 tons annually. To give a general idea of the produce of these 

 mines compared with others, it may be right to state that the 

 whole of the lead-mines of Britain may be considered to afford an 

 average annual produce of 45 to 50,000 tons. 



This district, exclusive of its metallic mines, is penetrated and 

 traversed by others, which, as they are only filled with lapideous 

 substances, are of little interest except to geologists, and to them 

 they have proved subjects of much discussion and some conten- 



