188 
ALLUVIUM AT BASE OF MOUNTAIN TORRENTS. 
I have seen a good example of these two deposits in Holland, 
in immediate contact with one another. The alluvial detritus of 
of rivers and mountain torrents, showing that the maximum of their force is wholly in¬ 
competent either to excavate the main trunks of the valleys through which they flow, or 
to produce the gravel beds that cover them at a distance from the hills and mountains 
whence this gravel has been transported, is given in chap. 20 of Dr. Kidd’s Geological 
Essays, and in the second Essay of Mr. Greenough’s Examination of the first Principles 
of Geology. The same subject has been treated with equal accuracy and ability by 
M. Brongniart in the latter part of his “ Histoire naturelle de l’Eau,” published in the 
14th volume of the “ Dictionnaire des Sciences naturelles,” and separately as a small 
pamphlet, which I strongly recommend to the attention of those persons who wish for 
correct information as to the effects produced by water upon the surface of our globe. 
A difficulty occurs frequently along the base of a mountain chain, in marking the 
exact line which separates the deposits of postdiluvian detritus, which have been and still 
continue to be drifted down by wintry torrents, from that gravel which is strictly of di- 
luvian origin. The bursting of an Alpine Lake (such as occurred in June, 1818, in the 
valley of the Dranse in Switzerland), and the daily action of torrents and rapid rivers in 
times of flood, are competent to produce partially over a limited district, beds of gravel 
somewhat similar to those of the great diluvian waters. Striking examples of this kind 
are afforded in the Duchy of Venice, along the base of that part of the Alps which lies 
immediately on the north and north-west of the town of Valvasone, where the flood waters 
of the Tagliamento, the Meduna, and the Zelline, have strewed the plains to an extent of 
many miles from the base of the mountains with a beach of pebbles of enormous breadth, 
which in summer is dry and barren, resembling a naked chesil bank on the sea shore. 
Similar features are presented by the Torre and Malina torrents on the north-east of 
Udina, and by the numerous torrents that descend into the plain of Lombardy, from the 
mountains on the north of Verona and Vicenza. The Trebbia and Taro rivers also, and 
all the torrents adjacent to them, which fall into the Po from the south, near Parma and 
Piacenza, cover the lands in the vicinity of their courses with a similar and annually in¬ 
creasing accumulation of detritus, from that part of the Apennines in which they take their 
origin. And in our own country, the small river of Avon Lwyd or Tor Vaen, which de¬ 
scends from the west side of the Blorenge mountain in Monmouthshire, by the valley of 
Pontypool, presents, at the point where it leaves the mountains immediately below that 
town, a naked strand of pebbles, that is perpetually shifting and laying desolate the level 
regions that succeed immediately to the sudden termination of the steep and precipitous 
mountain valley, along which the torrent has its course above Pontypool. 
At the point where transverse mountain valleys fall into the great longitudinal valleys. 
