92 Ohservatmis on the Geology 



of the contents of the vein being richer as it passes some of the 

 beds than in others. It would not have suited his theory to have 

 admitted more ; but the fact is, that so far from this being a rare 

 occurrence, it is almost a general law, at least in England, where, 

 I l)eHeve, mining operations have been carried on to a far greater 

 extent and with more capital and skill than in any part of Ger- 

 many. The principal substances which fill the veins in this 

 mining district, besides galena or the sulphuret of lead, and 

 blende or the sulphuret of zinc, are quartz, fluor spar, calcareous 

 spar, iron pyrites, and pearl spar, a soft unctuous iron ochre 

 which stains the fingers, and sulphate of barytes. Carbonate of 

 bar)tes and carbonate of lead more rarelv occur; copper pyrites 

 is found in some of the veins, but not in sufficient quantity to be 

 worked for that metal. Emerald green crystals of fluor spar 

 occur in some of these mines, more beautiful perhaps than any 

 that have elsewhere been met with. The carbonate of baiytes is 

 principally found in large detached balls, which have a radiated 

 diverging structure. I have seen some of them not less* than 

 ten inches in diameter. 



Mr. Forster relates a striking change in the barytic spar as it 

 passes through different strata at Welhope in Northumberland. 

 The vein in the sandstone strata contains sulphate of barytes 

 (caulk); but when it enters the great limestone, the carbonate of 

 barytes becomes the matrix. 



The cubic crystals of fluor are sometimes very large, and often 

 beautifully coated with minute brilliant rock crystals and with 

 pearl spar. The phaenomenon of quartz formed on cubes of 

 fluor which have subsequently disappeared, is frequent in these 

 mines, and has been long well known to English mineralogists. 

 A similar phaenomenon is observed in the mines of Derbyshire, 

 where calamine is found formed on the metastatic crystals of car- 

 bonate of lime, commonly called the dog tooth spar ; the crystals 

 have been removed by some \mknown process of nature. A si- 

 milar effect also sometimes takes place with cubic galena, and 

 the miners consider these operations as constantly going on ; to 

 use their expression, the " calamine is eating up the lead." 



The mineral repositories in this district are the Rake vein ; 

 the Flat vein, which is a lateral expansion from the Rake vein; 

 the Pipe vein, which is a flat vein compressed on its sides so as 

 to form a tubular ca\'ity ; and the Accumulated vein, in which a 

 number of veins converge, and unite, forming a kind of cone, 

 filled with ore and veinstone. A more particular description of 

 these will be given elsewhere. 



The whole district is intersected by numerous faults, or 

 dykes, which cut through and disturb the strata. Dyke in 

 the provincial language of North Britain signifies a u^a/// and 



these 



