12 PROFESSOR FORBES ON THE VOLCANIC GEOLOGY OF THE VIVARAIS. 
down (which is commonly due to the pressure of lava) on the south-west side. 
A very careful examination in 1841 changed my opinion; and, notwithstanding 
the seeming improbability of the conclusion, we must, I believe, admit that the 
lava of Jaujac terminates almost exactly where the lava of Neyrac commences, 
filling the valley to almost the same depth, and with similar matter. And this 
must be conceded for the following amongst other reasons :—/irst, There is a 
break in the continuity of the lava stream a little above, and opposite to the 
village of Souillols, and in the cliff formed by the river. The granite of the 
country may be traced in the interval. Secondly, Up to this point the character 
of the lava, as displayed continually in the cliff on the right bank of the stream, 
is remarkably uniform upwards from the point of section (fig. 2) opposite the 
Castle of Mayras. Only a small portion of the lower part is columnar, sur- 
rounded in the greater part of its thickness by basalt nearly amorphous or 
slightly columnar. The columnar part diminishes in thickness as we ascend 
the course of the river, and opposite Souillols it is only three or four feet in 
height. But when the lava cliff reappears after the break alluded to (a break, 
however, so slight, that it might easily escape notice), it presents a very different 
front to the river. The cliff is now 130 feet in height,* of which not less than 
two-thirds, and in some places nearly the whole is composed of a single range of 
perfectly continuous basaltic pillars; and the level of the prismatic boundary is 
again gradually depressed, as we approach the undoubted origin of this part of 
the stream, namely, the crater of Jaujac. Thirdly, The volcano of Neyrac does 
exhibit a streak of ashes and slag down its southern face. Now the ashes may 
be seen to pass into the slag, and the slag into the lava of the Alignon near 
Souillols,—a convincing argument. 
We now come to describe more particularly the lava of Jaujac, which ex- 
tends along the bottom of the valley, from a short distance above the village of 
that name, to nearly opposite Souillols, at least two miles farther down. All this 
space has been raised from the natural bed of the stream to a vertical height of 
perhaps 100 feet on an average, throughout the entire breadth of the valley, and 
now presents a cultivated and wooded plateau, whose extraneous origin would 
hardly be suspected but for the deep incision made by the river near the foot of 
the hills which bound it to the north. This section displays the wonderful colon- 
nade already referred to, of which so correct a representation has been given in 
Mr Scropr’s beautiful work on Central France. 
That representation exhibits well the remarkable fact of the gradual rise of 
the lower perfectly-columnar stratum into the higher or imperfect one, in pro- 
portion as we follow the section down the valley, or farther from the point of 
* Measured by the fall of a stone, and confirmed by the authority of a person who told me that 
he had measured it with a string. 
4 
