VOL. LXV.J PHILOSOPHICAL TRANSACTIONS. 579 



very rough, and sometimes knotty surface, but, when broken, show a varie- 

 gated colour and unequal texture of parts. They are commonly speckled, as it 

 were, more or less distinctly, and resemble an inferior sort of granite, of which 

 Monte Rosso itself is formed, and which serves as a base to the range of columns 

 in question. It is, in general, not quite so hard as the Alpine and Oriental 

 granites, and is sometimes even friable. Linnaeus justly observes, that this 

 species of granite abounds in France ; for I have lately seen large tracts of it in 

 the neighbouring provinces of Auvergne, Velay, and Lionnois ; and apprehend, 

 that it likewise abounds in the Vivarey, Gevaudan, and Sevennes mountains ; 

 from the affinity observable in the physical geography of those countries. But 

 it is equally common in Italy ; for besides Monte Rosso, the bulk of the Eu- 

 ganean hills in general, of which that is a part, principally consists of it ; and 

 these hills occupy a considerable tract in the plains of Lombardy. It is also 

 common in the Tuscan and Roman States : the mountain close to Viterbo, on 

 the road to Rome, is entirely composed of it. The columns of Monte Rosso 

 appear therefore of a different character from any hitherto described by mine- 

 ralogists, who only mention those of a uniform colour and texture. But the 

 great singularity here is, that such a range of prismatic columns should be 

 found bedded, as it were, in a mass of granite, and composed nearly of the same 

 substance ; of which I never yet saw or heard any other instance. This circum- 

 stance seems therefore to render the causeway of Monte Rosso more curious 

 and singular than the famous one in Ireland is known to be, from the regular 

 articulation of its columns ; the same phenomenon having lately been discovered 

 at Staffa, one of the western islands of Scotland. Different groups of articu- 

 lated basaltine columns have likewise been observed in the province of Auvergne 

 in France; particularly by M. Beost de Varennes, at Blaud near Langeac; and 

 by M. Desmarests, near le Mont d'Or. M. Sage also mentions another near St. 

 Alcon, in the same province. The Monte Rosso group is, however, not only 

 curious in itself, but very interesting, on account of the great light it seems to 

 throw on the origin of granites in general. 



It is remarkable, that the columns in the two different groups of Monte 

 Rosso and San Giovanni, preserve respectively the same position, nearly parallel 

 to each other; which is not commonly the case in other basaltine groups. Fpr 

 though the principal aggregate, which forms the Giants Causeway, stands in a 

 direction perpendicular to the horizon ; yet other small detached groups of 

 columns also appear in the hill above, that affect by their position, different de- 

 grees of obliquity. Among the numerous basaltine hills of Auvergne and 

 Velay, in France, which seem to abound in those provinces more than in any 

 other part of Europe, and perhaps of the known globe, nothing is more common 

 than to see the columns of the same group lying in all possible directions, as 



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