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GEOLOGICAL MEMOIRS. 



carboniferous systems. Besides these, plutonic masses (though not 

 of considerable importance) occur on both sides of the Esla within 

 the district of the transition formations on the north. At the base 

 of some parts of the southern transition range, conglomerates and 

 nagelflue occur extensively. 



The principal beds of the coal-formation consist of clay-slate, black, 

 grey, or red slaty clay, and quartzose sandstones, amongst which there 

 is a very remarkable cellular variety. The direction of the beds is 

 everywhere from east to west ; they are sometimes vertical, and some- 

 times dip to the south at an angle varying from 45° to 90°. They 

 contain abundant impressions of plants of the genera Calamites, Si- 

 gillaria, Lepidodendron, Lycopodites, &c. Some shells have also been 

 noticed. The number of the coal-seams is remarkable, although 

 workable beds are not found extending over the whole formation. 

 They differ much, not only in thickness and persistency, but also in 

 the constitution and goodness of the coal. The thickness of some of 

 the seams is astonishing, amounting to 50, 60, and even 100 feet. 

 That which is procured from the neighbourhood of the older forma- 

 tions is usually inferior to that which is raised at a greater distance. 



The transition rocks, forming the southern limit of the coal-field, 

 contain on their southern declivity a thick, dark band of limestone 

 studded with nodules of white calcareous spar, which alternates with 

 a grey or black clay-slate, occasionally containing mica, and efferves- 

 cing with acids. The limestone contains no fossils ; in the schist are 

 found traces of plants. In the rest of the southern range of hills the 

 prevailing rock is a grey, white, or sometimes red limestone, which is 

 accompanied by a yellowish grey slaty clay, and a white, grey, or red 

 sandstone. In these formations occur many fossil Brachiopoda, Cri- 

 noidea, and Corals. The strike of the beds in the southern chain is 

 universally from east to west. The dip varies, but is usually to the 

 south and always at a high angle. In some places a northern dip is 

 observable, and also a fan-shaped arrangement of the strata. 



These rocks, which are the prevailing ones in the southern chain of 

 hills, compose also the greater part of the range rising to the north of 

 the coal-formation. But here the sandstone, which to the south is 

 little developed, becomes more important. Few fossils are found in 

 it. The accompanying limestone is very cavernous. In some parts 

 of the formation, for instance in the Sierra de las Cuestas, a hard 

 sandstone occurs, which passes into compact quartzite. The lime- 

 stone does not constantly accompany it, but in its place is found 

 a dark or greenish slate-clay which does not effervesce with acids. 

 In this rock no fossils are found. The dip and strike of these strata 

 agree generally with those of the southern chain. The only exception 

 is the Sierra de las Cuestas, where the strike is north-east and south- 

 west and the dip from 40° to 45° to north-west. 



From the researches of Mr. Pratt there can be no doubt that the 

 fossiliferous transition rocks which form the limits of the coal-forma- 

 tion of Leon belong to the division which is styled in England the 

 Devonian system. Whether the rocks composing the Sierra de las 

 Cuestas be of the same age, or are to be considered as Silurian, is diffi- 



