28 Mr. N.J. Winch's Contributions to the 



flints contained traces of bitumen or some similar substance of 

 organic origin. To it the dark colour of flints was owing, and 

 to its destruction the whiteness of roasted and bleached flints 

 was attributable. 



The lecturer, in conclusion, briefly referred to the forma- 

 tion of some other minerals. He explained that the produc- 

 tion of crystals of selenite, celestine, and heavy spar, obviously 

 resulted in many cases from the sulphuric acid arising, one 

 while from burned sulphur in volcanic districts, and at another 

 from oxidizing pyrites, acting upon contiguous masses con- 

 taining lime, strontia, and baryta. He showed a specimen 

 of red oxide of iron possessed of a stalactitic form decisive of 

 aqueous origin ; and oxide of manganese, he said, sometimes 

 occurred in a similar state. He considered such specimens 

 to have been originally deposited in the state of carbonates, 

 out of solutions of carbonic acid, and to have been sub- 

 sequently still further oxidized, — a change which he illustra- 

 ted by a specimen of carbonate of manganese kindly given 

 to him by Mr. Phillips, in which the progress of conversion 

 was distinctly exhibited. He also suggested a possible ex- 

 planation of the origin of the pyrites so often found in fossil 

 shells imbedded in clay which abounds in nodular pyrites. It 

 had been observed that sulphates undergo gradual decom- 

 position by the action of organic matter ; and he thought it 

 therefore far from improbable that sulphate of iron, generated 

 from oxidized pyrites, might by the deoxidizing agency of 

 animal remains be reconverted into sulphuret. 



VIII. Contributions to the Geology of Northumberland and 

 Durham. By N. J. Winch, Esq. G.S. <$• A.L.S* 



1V/TINERS and geologists at all conversant with the great 

 ^ •*■ coal-field of Northumberland and Durham, are acquaint- 

 ed with the vein generally called the Main or Ninety-fathom 

 Dyke, which traverses the district in a south-westerly direction, 

 dividing it into two unequal portions, and which is only re- 

 curred to here as a guide to the situations of the coal mines 

 whose sections form the subject, and are the chief value, of the 

 following paper. This dyke may be seen to advantage both 

 in the cliffs and intersecting the rocks on the sea-shore a little 

 to the north of Cullercoates, from whence it passes by Whitley 

 Quarry to Earsdon, Backworth, Killingworth, Gosforth, Den- 

 ton Hall ; by the north corner of the field east of West Den- 



* Communicated by the Author. [The reader of these contributions 

 should consult the map of Northumberland and Durham, in the First Series 

 of the Transactions of the Geological Society, vol. iv. Plate I. — Author] 



