10 



GEOLOGICAL MEMOIRS. 



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The following section shows the order of succession and the strati- 

 graphical relations of these several systems : — 



This section has been 

 already given by Mr. D. 

 Sharpe in his paper " On 

 the Geology of the Neigh- 

 bourhood of Oporto " * ; it 

 represents the relations 

 which apparently exist 

 between the foregoing 

 systems of rocks and the 

 coal-beds, the latter ap- 

 pearing to dip beneath the 

 former. 



Even if the manner in 

 which the coal-beds here 

 crop out intimated their 

 true geological situation in 

 reference to the rocks of 

 the fourth system, which 

 appear to cover them, it 

 would still remain tobe ex- 

 plained why the mining- 

 works that have been car- 

 ried out in the neighbour- 

 hood of S. Pedro da Cova, 

 Covelo, and Povoa have 

 always been in that zone 

 in which the valuable beds 

 are exposed, and why the 

 attempt has never been 

 made further east, in 

 which direction the coal 

 dips at an angle of about 

 35° under the coal-shale. 

 If it had been discovered, 

 through the first work- 

 ings, that the coal-beds 

 continued further beneath 

 and dipped under the Si- 

 lurian beds, it would have 

 been attempted to win the 

 coal again by shafts sunk 

 to the east of the inter- 

 mediate zone. As this, 

 however, has not taken 

 place, there is every rea- 

 son for doubting this con- 



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* Quart. Journ, Greol. 

 vol. v. 1849, p. 145. 



Soc. 



