404 Mr. Ainsworth on the Veins of Leadhills, ^c. 



tions, which now occupy our attention, and in which clay-slate and 

 greywacke are to be seen over considerable spaces, Avithout being 

 accompanied by black limestone. 



The two varieties of clay-slate, distinguished by being of a sil- 

 very lustre and steatitic character, passing into talc-slate, and the 

 green clay-slate, in very thin laminae, highly carburetted, may al- 

 ternate or succeed to one another, as in the mines of Mellada, An- 

 unas, and Payas ; but there can be no doubt, from the observations 

 of Charpentier, that greywacke and clay-slate are in general more 

 ancient than black limestone. When nearest to the primitive for- 

 mations, the latter is subordinate, while in the newest part of the 

 transition formation, clay-slate is only found subordinate to lime- 

 stone. 



When we consider the frequency of these associations, and the 

 periodical alternation, or the development of a partial member of 

 the same association enveloping or reducing one another (by an un- 

 equal increase of bulk) to the state of simple subordinate beds, we 

 are not astonished at this partial development of one or two terms 

 in the series. Thus the transition formations of the Cotentin and 

 Alps of Switzerland, contain no porphyry ; the same formations in 

 the Venezuela contain neither greywacke nor porphyry. In the 

 Pyrenees they contain no granite nor sienite. In Hungary, accord- 

 ing to the observations of M. Beudant, they also contain no grey- 

 wacke. 



Thus, if we leave the consideration of the types of local super- 

 position, to resolve these formations into groups by geognostical 

 generalization, we shall at once find that the transition rocks of the 

 counties of Dumfries, Lanark, and Selkirk, reposing in their south- 

 western extremities on primitive rocks, belong to the same great 

 formation of clay-slate that crosses the Western Pyrenees, the 

 Alps of Switzerland, between Hartz and Glaris, and the north of 

 Germany from the Hartz as far as Belgium and Ardennes. It 

 is the clay-slate of the Cotentin, Brittany, and Caucasus. It in- 

 cludes the schistose rocks in Norway, placed below the porphyries 

 and green sienites. It is, according to De Humboldt, the green 

 clay- slate of Malpasso, in the Cordillera of Venezuela, and the 

 clay-slate in the sienites of Guanaxuato, in Mexico. It is at once 

 distinguished from the transition sienite and porphyry of Snowdon 

 and Ben Nevis ; the greywacke and transition limestone? of May 

 Hill, Longhope, and of North Wales ; and it is equally well dis- 

 tinguished from the greywacke, phonolites, argillolites and argillo- 

 phyre (claystone and claystone porphyry,) of the Pentland Hills. 

 Professor feedgewick is, I understand, engaged in investigating 

 the mineralogical structure of the Cheviot Hills, with outlying 

 phonolitic domes, (sources of the Jed ;) and I have every certain- 

 ty that his researches will throw additional light upon the mineral- 

 ogical constitution and geognostic age of these neglected mountain 

 chains. . - . 



