238 



Murchisoris Silurian System. 



whose mining implements are found in them. In one of the 

 principal veins, says Mr. Murchison, the wall is granular 

 felspar, with green earth; and two contiguous bosses of 

 crystalline greenstone rising up unconformably through the 

 strata of sandstone and shale, cut off the productive veins. 

 In this case, Mr. Murchison is of opinion, that there has not 

 been a sufficient quantity of contiguous sandstone and shale to 

 afford ground for the production of the ore. The altered 

 rocks in which the only instances of metalliferous veins 

 occurred to his observation throughout the Silurian rocks, 

 were situated in the vicinity of intrusive trap, and that the best 

 veins of galena were found in the lower Silurian rocks near 

 the great outburst of trap. In the shales and sandstones con- 

 nected with the altered rocks in which the veins of galena are 

 situated, Mr. Murchison found Trilobites,* identical with 

 those of the lowest flags of the Silurian system, together with 

 many shells of the Caradoc sandstone, and thus even in altered 

 disrupted strata he was able to discover the true nature of 

 the rocks in which metallic veins occur in distant situations. 



Three of the lead mines situated in lower Silurian rocks 

 of Shropshire, described by Mr. Murchison, near the vil- 

 lage of Shelve, afford the following produce : — 



Bog mine, 1,554 tons of lead, 



Snail batch, 1,300, ditto, 



Grit and gravel mine, 685, ditto. 



Total, 3,539 

 The present depth of the engine shaft of the Bog 

 mine is 293 yards, and the lowest working level 265 yards, 

 the upper working above the adit or boat level 105 yards. 

 In this district, Mr. Murchison observes, there are thirty 

 veins that have been profitably worked. 



The Stiper Stones, is a barren naked ridge of quartz rock 



* In a future number we hope to give examples of these curious 

 fossils from Mr. Murchison's work. — Ed. 



