Professor Forbes on the Geology of Auvergne. 17 



point of time, as well as in effect, to those which the elevation 

 theory assumes. 



5. I have already said tliat the theory of the tufas and con* 

 glomerates of this country appears so difficult, that I should 

 be unwilling to found theoretical conclusions upon them. Their 

 stratification, such as, in some cases, I firmly believe to exist, 

 affords, however, an evidence too important to be overlooked. 

 The section formed by the river Cer near Tiesac (in the Cah- 

 tal) is one of the best instances. The following memorandum 

 was made at the time : " I crossed the valley to examine a sec- 

 tion of the constituents of the mountain, exposed by a very ex- 

 tensive and picturesque landslip. Above the conglomerate or 

 tuff which forms the bottom of the valley, is a bed of compact 

 trachyte, very like the claystone porphyry of the Pentland Hills. 

 It has a tendency to columnar structure transverse to the direc- 

 tion of the bed, and breaks with the hammer also in that direc- 

 tion, though the texture of the rock indicated by the crystals is 

 parallel to the bed. Above this bed is one of tufaceous conglo- 

 merate, like those of Naples, and strongly appearing as if stra- 

 tified by water ; then another bed of compact trachyte ; then 

 coarse conglomerate ; and the whole is capped by basalt (which, 

 hovvever, I did not ascend high enough to meet with). The lowest 

 bed is of great thickness. It is extensively exposed in the chasm 

 below Tiesac, through which the river runs. It is a matter of 

 question whether this chasm is of erosion, or a crack."^ Now 

 the whole of these beds dip at an angle of 3° 42', at a mean, ac- 

 cording to M. Elie de Beaumont, though in many places consi- 

 derably more, from the vertex of the cone. If the third bed from 

 the bottom which is conformably stratified really indicate aque- 

 ous deposition, the conclusion is nearly irresistible that such de- 

 position took place whilst the bed of trachyte below was hori- 

 zontal ; and hence, by the reasoning of the last article, the beds 

 above must also have been subsequently horizontally deposited. 



The importance of the question I lay before future observers, 

 nor shall I take upon myself to make any general statement r^ 

 lative to these tuffs, considering the short and imperfect exami- 

 nation which I gave them, and the difficulties which at the time 

 I felt upon the subject. My observations in the valley of the 

 Cer were, however, carefully made, and were entirely confirmed 



VOL. 5CXI. NO. XLI. JULY 1836. B 



