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limestone rocks they traverse. At Mucruss mine, in the same neigh- 

 bourhood, copper ore was obtained from a true metalliferous bed. 

 In Kenmare the deposits of lead ore are shown to be discontinuous 

 masses, nearly parallel in range and dip to the regular strata. 



In the county of Cork the most valuable mine of copper is opened 

 in a true vein : but the author remarks that in some parts of this 

 county there is a very general diffusion of cupreous matter, some- 

 times appearing in separate particles, and sometimes in strings 

 veins or filaments more or less connected with each other, but not 

 continuous, and therefore contemporaneous with the rocks to which 

 they are subordinate. Such repositories of metals might not inaptly 

 be termed "veins of segregation," as they seem to have been formed 

 by a separation of parts during the gradual passage of the mineral 

 masses into a solid state. 



In England we have almost every variety of metalliferous deposits. 

 Near Whitehaven in Cumberland great masses of reniform hematite 

 alternate with red beds of mountain limestone. At Nosterfield, near 

 Bedale, a true bed charged with sulphuret of lead alternates with the 

 upper strata of magnesian limestone. The great copper pipe veins of 

 Ecton must have been contemporaneous with the shale limestone 

 to which they are subordinate. The great lead veins of our northern 

 counties originated, if I mistake not, in cracks formed during the ele- 

 vation of the carboniferous chain, before the period of the new red 

 sandstone. 



In Cornwall we have, as is well known, both on the great scale 

 and the small, every modification of veined structure. Tin is dis- 

 tributed through some of the granitoid rocks where no vein is visible. 

 The slate rocks, near their junction with the granite, are traversed by 

 veins of injection, and some of these are metalliferous, (for example, 

 an elvan or porphyry dyke near St. Austell). The regular metallife- 

 rous lodes were probably once but cracks and fissures produced du- 

 ring some periods of elevation ; and how they have been filled up is 

 perhaps a question beyond our scrutiny. But after the important ex- 

 periments of Mr. Fox, there can, I think, be no doubt that the great 

 vertical dykes of metallic ore, which rake through so many portions 

 of the county, owe their existence, at least in part, to some grand de- 

 velopment of electro-chemical power. 



In all the crystalline granitoid rocks of Cornwall there are also 

 many masses and " veins of segregation." Such are the great contem- 

 poraneous masses and veins of schorl rock ; and some of these are 

 metalliferous. The decomposing granite of St. Austell Moor is tra- 

 versed, and sometimes entirely superseded, by innumerable veins of 

 this description. Upon these lines of schorl rock there is often aggre- 

 gated a certain quantity of oxide of tin, which sometimes diffuses itself 

 laterally into the substance of the. contiguous granite. After examin- 

 ing this district with Professor Whewell during the summer of 1828, 

 we left it in the conviction that several of the neighbouring tin works 

 were opened not upon true lodes, but upon "veins of segregation." I 

 only throw out these remarks as hints for future inquiry ; as the sub- 

 jects introduced by the memoir of Mr. Weaver are of vast importance, 



