Geology 



study the rocks of the country, and it was during his 

 wanderings in Auvergne that he was struck by the 

 strange black columnar basalts of that region. 



Basalts had been known and described in many parts 

 of Germany and at the Giants' Causeway in Ireland, 

 but up to the time of Demarest, basalt and lava had 

 not been connected. In Auvergne, on the edge of the 

 plateau of Prudelle, he found a part of an ancient lava- 

 stream isolated by denudation and showing columnar 

 structure, and afterwards observed the same structure 

 in undoubted lava flows definitely connected with 

 the Puys, as the extinct craters of that district are 

 called. ' 



He was thus led to believe that basalt belonged to 

 the class of volcanic products. " I draw from this recog- 

 nized resemblance, and the facts that establish it, a 

 deduction which appears justified by the strength of 

 the analogy namely, that in the Giants' Causeway, and 

 in all the prismatic masses which present themselves 

 along the cliffs of the Irish coast, in short, even among 

 the truncated summits of the interior, we see the 

 operations of one or more volcanoes which are extinct, 

 like those of Auvergne. Further, I am fully persuaded 

 that in general these groups of polygonal columns are 

 an infallible proof of an old volcano, wherever the stone 

 composing them has a compact texture, spangled with 

 brilliant points, and a black or grey tint." 



It was fully two years after his memorable visit to 

 Auvergne that Demarest communicated his results to 

 the Academy of Sciences at Paris, and not wishing to 

 be precipitate in the publication of his views, it was by 

 his wish that the paper did not appear in the Memoirs 



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