VISIT TO EUROPE: RESIDENCE IN EDINBURGH. 171 



Salisbury Craig is situated in relation to Edinburgh, almost 

 exactly as the East Rock is in reference to New Haven, 

 and the two are not unlike in form. I have already men- 

 tioned the Castle Rock as trap or basalt. Arthur's Seat, 

 also very near to Edinburgh, is nearly eight hundred feet 

 high, and in elevation and form is not unlike our Mount 

 Carmel ; and between the old town of Edinburgh and the 

 new, or rather on the borders of the new, rises the Calton 

 Hill of porphyry, the hill which is consecrated to mon- 

 uments, those of Hume, Sir Walter Scott, and others. 

 I now felt that my geological difficulties were vanishing, 

 and I began to repose, in great confidence, upon the double 

 action of fire and water. After my return home it was a 

 great pleasure to me to view all the trap ranges of Con- 

 necticut and Massachusetts and New Jersey, as belonging 

 to the same category with Scottish trap, or whinstone, as 

 it is called in Scotland. I became convinced, also, that the 

 basalt of the Giant's Causeway belonged to the same family ; 

 and that compact lava and trap basalt and porphyry, 

 are merely modes of one and the same operation. Many 

 years afterwards, (May 1851,) on Mount Etna, I saw true 

 basaltic columns that had been formed in a lava current 

 (See Visit to Europe in 1851, Vol. II. p. 26). Extended 

 observations in different countries, and comparison with 

 Vesuvius and Etna, by a visit to those mountains, have 

 given my mind entire satisfaction on the subject of igneous 

 agency. Seventeen years after my return from Scotland 

 Cordier's little book appeared, assuming to prove, as he did 

 prove, that the heat increases regularly in all countries as 

 we descend into the earth, after passing below the effects 

 of atmospheric variations ; and the average rate of increase 

 is about one degree for every fifty feet of descent. Of 

 course, if the ratio, or any ratio of increase, continues, we 

 must eventually arrive at ignited and even melted rocks. 

 The deductions of Cordier have been since confirmed by 

 observations made in many countries, particularly in deep 



