Igneous Origin of some Trap Rocks. 121 



of Fingal, in the island of Staifa, and the Giant's Causeway, in the 

 xiorth of Ireland, are still more signal examples ; on account of the 

 regularity and height of the columns, and of their curiously jointed 

 structure. The ironstone is the usual name of the common people 

 for our trap, and this name has allusion, no doubt, to the dark color 

 and great weight of many of the trap rocks. 



Two opinions. 



There has been much discussion among geologists on the question 

 whether these rocks are of aqueous or of igneous origin. The Wer- 

 nerian school have maintained the former opinion, and D'Aubuisson, 

 one of its ablest disciples, after writing an interesting volume* to prove 

 it, was led by an examination of the ancient volcanic district of Au- 

 vergne, Velay and Viverais, in France, to change his opinion. The 

 Huttonian and, generally, the Igneous geologists have sustained the 

 opinion that the trap rocks have originated from fire ; and the pro- 

 gress of investigation has done much of late years to estabhsh it. I 

 was accustomed to hear these subjects discussed with great interest 

 and ability, at Edinburgh, by Dr. Hope on the Huttonian and Dr. 

 Murray on the Wernerian side. Prof. Flayfair had also at that time 

 published his Illustrations, and Dr. Murray his Comparative View. Tf 

 I was not convinced, I was always entertained, by these learned and 

 luminous teachers ; but without taking sides, I reserved myself for 

 opportunities of future observation. I used indeed, at Edinburgh, to 

 see the sandstone under the trap of Salisbury Craig, f apparently hard- 

 ened, as if by the action of fire ; and the Huttonian challenged tliis 

 appearance as a proof in his favor ; while the Wernerian claimed 

 that it was an alteration produced simply by drying. 



Rocky Hill. 



Trap rocks abound in this country, and the two principal ranges 

 already cited, with Vvhich I am personally acquainted ; (those of New 

 England and New Jersey,) repose upon sandstone, commonly con- 

 sidered as the old ; but embracing, in different places, many varie- 

 ties, from graywacke through conglomerate and puddingstone, to 

 the micaceous and argillaceous sandstones. J Junctions of different 



* D'Aubuisson on Basalt. 



t Which much resembles the East Rock at New Haven, and the two trap ridges 

 maintain nearly the same position in relation to the respective towns. 



t As more fully described by Prof. Hitchcock, in his account of the Geology, &c. 

 of the valley of the Connecticut, Vol. VI, p. 201 of this Journal, 



Vol. XVII.— No. L 16 



