VALLEY OF THE ALIGNON—JAUJAC. 11 
and will shoot outwards, so as to form simultaneously at points equidistant from 
that surface. The accuracy of the empirical law is greater than any theoretical 
view could possibly lead us to expect. Varied examples will be quoted in the 
following pages. 
A little way above La Beaume, near the junction of the rivers of Ardéche and 
Fontaulier, is the picturesque Castle of Mayras, which, however, offers in itself 
nothing of much interest. Opposite to this castle (on the other side of the 
Ardéche) the lava cliff continues from the point already described, so that the 
colonnade of La Beaume owes its origin to the western, and not to the eastern 
valleys which unite there. There is little doubt, however, that the lava stream 
of the eastern valley (or that of Montpezat) may be traced at the sharp turn of 
the road to Thuez, opposite the Chateau de Mayras, where there is evidently an 
older and inferior stratum of lava below the greater colonnade. A section at this 
point, of a cliff about 100 feet in height, is given in fig. 2. The bed No. 3, I take 
to be the lava of the valley of Montpezat. It contains a remarkable quantity of 
olivine, and is here amorphous: it is completely detached from the superior mass, 
which is beautifully columnar where it rests upon the older bed, the columns 
being vertical, since they rest upon a horizontal base. The beds Nos. 1 and 2 
might appear also to belong to distinct currents, the upper part shewing but im- 
perfect columns ; a close examination shews, however, that the beds (1) and (2) 
inosculate in such a manner as to leave no doubt of their being due to one and 
the same eruption, and that the distinction is caused by the accidental manner 
of their consolidation. 
Valley of the Alignon—Jawac. 
A short distance higher up, the Ardéche divides from its tributary the 
Alignon, the former descending from Thuez, the latter from Jaujac. The great 
mass of the lava which we are tracing evidently descends from the latter valley, 
and here we gain the first view of one of the volcanic vents which has furnished 
the lavas of the lower valleys, but presents from this point no trace of a crater. 
It is situated on the ridge separating the valleys of the Ardéche and Alignon, and 
is sometimes called the Volcano of Neyrac, from a village of the former, or Souil- 
lols, from one of the latter. And here occurs an interesting question, whether 
the great single lava-flow, extending to the Pont de La Beaume, is due to this 
volcano, or to that of Jaujac, higher up the Alignon ; for it undoubtedly did not 
come from the valley of Thuez (on the Ardéche). On my first visit in 1839, I 
was of opinion that the stream might be traced uninterruptedly from the volcano 
of Jaujac, and that there was no sufficient evidence that the volcano of Neyrac 
had yielded a considerable stream into the valley of the Alignon, notwithstanding 
the presumption afforded by its crater (a considerably decayed one) being broken 
