Reviews— Tyndall and Ramsay on Erosion. 271 
periodical convulsions on a grand scale to have been concerned in 
the elevation of mountains and the formation of valleys, has recently, 
from the Chair of the Geographical Society, repeating his firm con- 
viction that his own views are correct, protested against the more 
modern theory.* 
Professor Tyndall’s paper is, like others by him on the same and 
similar subjects, clear, definite, and positive. It brings forward his 
experience of the present year, and points out its bearing on what 
he has before seen and described. Two very remarkable longitudinal 
and transverse valleys, the gorge of Pfiffers and the celebrated Via 
Mala of the Spliigen Pass, both quoted by the advocates of the 
Fissure-theory in their own favour, were visited by him for the 
express purpose of testing the Erosion-theory. Certainly there are 
few phenomena of the kind in Europe more striking. In the Via 
Mala the road is an artificial ledge on one of the vertical walls of a 
deep cleft. One looks up to a narrow strip of sky overhead, and 
down on a roaring torrent, scarcely heard in the far depth. In the 
gorge. of Pfaffers the scene is, if possible, more striking; the rocks 
meet, or seem to meet overhead, and the valley is still narrower. 
Yet it is certain that on the walls of these two valleys, from bottom 
to top, there are unmistakable marks of erosion. Whatever may 
have been the cause of the cleft the water has always been there to 
take advantage of each widening of the split. Water has been 
present too at the top to act at the first commencement, and rush into 
the opening formed. ‘The two valleys are (if we remember right) 
nearly at right angles, and open into the valley of the Rhine near 
Coire. 
Professor Tyndall points out that such narrow gorges are chiefly 
confined to limestone. They are certainly not always so, though in 
limestone they are common. Limestone is a rock that has a special 
tendency to crack systematically, and its joints facilitate greatly the 
action of water. We know of similar crevices in granite, and in that 
rock it is generally the existence of veins of softer mineral that 
determine the action of water. It may be so in limestones and else- 
where; but in most of these cases there is a certain amount of 
undermining that rapidly increases the depth, but also widens the 
crevice. It is not necessary, however, that there should be a crack 
of any importance produced during elevation or by disturbance to 
determine the commencement of a deep valley, for the remarkable 
Cafions on the Colorado River (described by Dr. Newberry) are 
clearly the result of erosion in horizontal strata; and something of 
the same kind on a smaller scale is common in the South of 
Spain. 
Mr. Tyndall points out that the cracks, if the result of disloca- 
tion, ought to extend through the cols, which are generally very 
different from the throat or gorge that the name implies. There is 
almost always a flat space at the top of a deep gorge, such as the 
Via Mala runs through, and on the other side a corresponding deep 
* See Gnonoeican Magazine, No, 3, p. 127. 
