GEOLOGY OF DERBYSHIRE. 235 



stone, in Derbyshire, nothing, as I said before, can be known, as 

 but little of it is anywhere visible. Professor Phillips, however, 

 does not assign a greater general thickness than from 500 to 1500 

 feet for this formation : it is probable, therefore, that more than half 

 the whole thickness of the mass may be seen in one part or other 

 of the Derbyshire district. 



The most valuable product of the mountain limestone is lead ore. 

 This is found in veins, which are of two kinds, technically called 

 " rake veins" and " pipe veins." Rake veins may be best under- 

 stood by conceiving a vertical, or nearly vertical, fissure in the lime- 

 stone, varying in width from a few inches to three or four yards, 

 and running in a directly straight line across the country, sometimes 

 for several miles. This fissure is filled with spar of various mine- 

 rals, and contains galena, or sulphuret of lead, either in detached 

 cubical crystals, or more commonly in great " ribs" in the spar. 

 Pipe veins are similar masses of spar and lead lying between the 

 beds of limestone, but they are generally connected more or less 

 directly with rake veins, or with great hollows and caverns in the 

 solid limestone. The rake veins run in certain directions, and have 

 others crossing them more or less nearly at right angles, and are 

 ultimately connected, as we shall see hereafter, with the dislocations 

 of the rocks. Where two rake veins cross, or where a rake vein 

 meets a pipe, the ore is generally very abundant. It is a remark- 

 able fact, that, on tracing a rake vein down to the toadstone, that 

 rock is found to interrupt it, and it was for a long time supposed 

 that the toadstone entirely " cut off the vein." It is now known, 

 however, that on piercing the toadstone the same vein may be re- 

 gained, and that it is frequently richer* than it was above. Veins 

 of spar, called " leaders" may be traced through the toadstone ; and 

 in some instances a lead vein has been worked for as much as ten 

 yards into the toadstone itself. This, however, is very rare. 



Organic Remains op the Mountain Limestone. 



These are, in some parts, very abundant, but belong almost 

 wholly to marine animals. Some plants and leaves may occasion- 

 stone. It is possible that the difference may be caused by the inclination of 

 the beds, which would, in a perpendicular shaft, increase their apparent 

 thickness. 



• A notion prevails (whether well or ill founded I cannot say) in Derby- 

 shire, that no vein is ixry rich till it sets covered cither by shale, clay, or 

 loadstone. 



