Formations on the Left Bank of the Rhine. 159 



ating on these parts into the basalt, are inclosed in great 

 abundance by this scoriform lava rock. 



At the bottom of the valley it becomes evident that the 

 mountain torrent called the Isbach has cut through and car- 

 ried off the greater part of the basalt streams which once filled 

 its channel to a considerable height, throughout an extent of 

 more than a mile above, and rather less than this below the 

 village of Bertrich. Patches only of basalt are left now on ei- 

 ther side of the present bed of the river, and most usually in 

 the concave elbows of the valley, but of these some present 

 cliffs fifty feet in height. The lower part of these masses of 

 basalt is regularly columnar, the columns being divided by 

 frequent joints, from two feet to six inches apart. Where they 

 have been long exposed to erosion from the torrent, the angles 

 of these short prisms yielding sooner than the nucleus, the co- 

 lumns appear formed of rude and flattened spheroids piled 

 upon one another. This is, in short, an example of the co- 

 lumnar divisionary structure passing into the globular, by the 

 increase of the number of joints. An arched passage, which 

 goes by the borrowed name of Fingal's Cave, nearly a mile 

 above Bertrich, exhibits this structure in the most perfect 

 manner. It has evidently once formed the channel of the lit- 

 tle torrent which now runs on one side of it, and which has 

 thus partly worn away the columns, till they are reduced to 

 mere piles of balls. 



The eruptions of these three or four contiguous vents were 

 probably simultaneous, or very nearly so. The lava streams 

 produced by them can be, with difficulty, distinguished from 

 each other, all uniting in the valley below, and the basalt of 

 all is identical in mineral character. It seems probable that 

 the thermal springs of Bertrich-bad owe their warmth to hav- 

 ing percolated through some mass of lava not yet quite cooled 

 in the interior of the schist rocks, occupying perhaps the pro- 

 longation of the fissures through which the lava streams were 

 expelled. It may be presumed, indeed, that the temperature 

 of these springs is diminishing in consequence of the gradual 

 cooling of this mass. It is at present below blood heat, but 

 appears, by its ancient celebrity, to have been formerly much 

 higher. Since the year 1773 it has not, I believe, been ana- 



