DYKES AND MINERAL VEINS IN ENGLAND. 415 



by the intrusion of granitic and porphyritic rocks ; the Ardennes 

 mountains, which yield few veins, develop hardly any igneous 

 rocks. 



The carboniferous limestone tracts of Mendip, Derbyshire, and 

 Flintshire, of Wharfdale, Swaledale, and Alston Moor, have been 

 shaken to pieces by many convulsions, and they are very rich in 

 lead and zinc ; but the greater part of the Yorkshire and Northumber- 

 land limestones, affected by only one or a few general elevations, 

 are poor in metal. 



In Newer Rocks. The newer rocks are metalliferous only in the 

 vicinity of the foci of their disturbance, as round the central granite 

 of France, near the igneous masses of the Pyrenees and the Alps ; in 

 all which places the metallic ores are PO related to the igneous rocks 

 that they occur only in a narrow zone at the junction of the igneous 

 and the altered stratified rocks. 1 



Conclusions on this Subject. We must not shut our eyes to 

 some decided differences between the situations of dykes and veins. 

 For instance, the Island of Arran is traversed by hundreds of dykes 

 of basalt, porphyry, and pitchstone, but metallic veins are almost 

 unknown there. Alston Moor is dissected like a map by veins of 

 lead ore, but very few whin dykes occur there ; on the contrary, in 

 Northumberland and Durham whin dykes abound in the coal tracts 

 where lead is hardly known. It is, besides, too remarkable a thing 

 to be overlooked, that south of Durham barely a solitary whin dyke or 

 porphyry dyke is known through the metalliferous tracts of York- 

 shire, Derbyshire, Somersetshire, and Flintshire. This contrast is 

 the more remarkable in the country about the sources of the Tyne 

 and Tees, because there basalt has been erupted in vast quantity, and 

 at its eastern termination appears related to several dykes of great 

 extent. This mass of basalt is traversed by the veins in the same 

 manner as the limestone is, and we may accept the conclusion that 

 both are due to heat, but that the vein marks a later or collateral 

 stage, in which there was less heat and more water, acting more 

 slowly. 



Mining. At the present day we find in California and Aus- 

 tralia the very same modes of working alluvial deposits which were 

 practised in Gallicia for gold, and the Cassiterides for tin in the 

 days of Pliny and Strabo and Herodotus, 2 and perhaps in equally 

 ancient times in those hyperborean regions, the Ural mountains, which 

 still furnish so much gold to Europe. Probably many great mountain 

 chains, full of quartzose and metamorphic rocks, yielded gold in the 

 earlier ages of the world ; though none of the rivers washed it out 

 in such abundance as the streams of Lydia. Not much of the gold 

 and silver of the ancient world was obtained by mines properly so 

 called, at least in Europe, till the heroic spirit was replaced by great 

 commercial activity. Then the Athenians dug silver ores from 

 Laurion, the Carthagenians and Komans obtained silver and gold 



1 Observations of Dufrenoy, Von Buch, &c. 

 - Hist. Nat., many notices. 



