few instances where the subsequent extension of one stream has caused the diversion of another.

Under ordinary circumstances, the valleys once formed are not obliterated by any concomitant changes, but exist as long as the hills remain from which the streams descend, and they are continually enlarged, either vertically or horizontally, by the action of the rivers that run through them, and the rain-water which runs into them.

CHAPTER III.

PROOFS OF THE AMOUNT OF MATERIAL REMOVED FROM EXISTING SURFACES.

IN

N this chapter we shall consider some geological features which furnish absolute proof of the enormous amount of detrition and denudation that has taken place in some parts of the country; these considerations will enable us to realize what vast thicknesses of rock have in many cases been removed during the process of erosion, which has led to the development of the present physical features of the country. They may be treated conveniently under the following heads:-1, evidence from the outcrop of strata; 2, from faults; 3, from outliers and inliers; 4, from mountains of upheaval.

1. Outcrop of Beds.-Where a series of beds dip one beneath another, as in fig. 152, over a considerable breadth of ground, it is clear that they cannot have originally terminated in that manner, for it is the very fact of their abrupt truncation that causes them to crop out on the present surface. The higher beds must originally have extended for long distances beyond their present lines of outcrop, and a very great thickness of rock must once have existed above the present surface.

To take an instance which has already been partially described, namely, the south-east part of Ireland, see figs. 122 and 123. There is reason to believe that the coalmeasures, which now only occur as isolated patches, had originally a very wide extension over this part of the country, overspreading and overlapping the Carboniferous limestone, as shown in fig. 125. So great, however, has been the loss of material during the process of detrition and denudation by which the present physical features have been produced, that not only have the coal-measures

been removed from the greater part of the country, but the erosion has in many places extended through the whole thickness of the Carboniferous limestone, and has cut deeply into the Old Red Sandstone below.

In some places the erosion has extended even to the bottom of this division, so as to re-expose the old denuded surface of the Silurian rocks upon which the sandstones were deposited. But it has gone even further still, for in some localities, as at Freagh Hill and along the northern slope of the Galtee Mountains, it has worn deep hollows and valleys in the Silurian rocks themselves, several hundred feet below that surface on which the Old Red Sandstone was deposited. From the extent of this denudation,

D

1 2 3

Fig. 155. Thickness of Rock removed by Denudation.

therefore, it can be proved that a thickness of between 5,000 and 6,000 feet of rock has been removed from some parts of the district.

The time required for the removal of such enormous masses of rock must have been very great; and equal volumes of rock must at the same time have been deposited elsewhere, because detrition and deposition are coordinate terms, one being the measure of the other. Accordingly, in Antrim and other parts of Ireland we find some of the deposits which were formed out of the materials supplied by the detrition of these older rocks.

Where a surface has been formed across anticlinal and synclinal folds, the outcrops of the different beds supply a means of actually estimating the minimum thickness of rock which has been removed since the folds were formed. Thus, in fig. 155, the distance between c and D may be several miles, and the thickness of the beds, 2, 3, 4, 5, may

be respectively 100, 200, 400, and 200 feet. It is clear that all these beds must originally have been continued over the anticlinal, A, or they could not have been brought into the synclinal trough, s. The minimum thickness of rock, therefore, which has been removed from the surface over the point, A, must be 900 feet; and if any more beds came in with the same dip beyond D, their thickness would have to be added to this amount.

Fig. 145, p. 435, illustrates an actual case in which this estimate may be made; if the thickness of the slates between B and D be 5,000 feet, it is clear that a depth of rock of more than that thickness must have been removed from the top of the anticlinal fold, B, and, further, the same thickness of rock must once have extended across the Menai Straits, and all over Anglesea, unless the strata thinned in that direction. That much of it really did so extend is proved by some of the beds being brought in here and there by faults and synclinal curves.

In the same way Sir A. Ramsay has shown that in some parts of South Wales a thickness of at least 11,000 feet of rock has been removed from the present surface of the country.1

2. Faults. Equally convincing is the evidence furnished by the great dislocations or faults which traverse every country. Some of these are vertical displacements to the amount of several thousand feet (see p. 364), and yet they seldom produce any effect on the surface of the ground. To take an extreme case as an illustration, there is a fault traversing the Appalachian Mountains in North America, which has a vertical throw of 20,000 feet, and yet the surface is so planed down that a man can stride across the line of fault, with one foot on a bed which was originally 20,000 feet above that on which the other foot rests. Again, many of the maps of the "Geological Survey of Great Britain," especially those of the coalfields, show so many faults that they are covered with a net-work of white lines. These faults are of various magnitudes, from a few feet to many hundred, and yet no sign of such displace"On the Denudation of South Wales," Mem. Geol. Survey, vol. i.

p. 297.

ments is visible at the surface, which may be a continuous tract of undulating fields and meadows. In all these cases it is clear that a thickness of rock equal to the maximum amount of vertical displacement has been removed from the surface of the country.

3. Outliers and Inliers.-An outlier is, as the name implies, an outlying or isolated portion of a bed, which has been separated from the main mass with which it was originally connected by erosion and denudation, just as an island is separated from the mainland (see fig. 122, p. 381, where outliers of Old Red Sandstone are shown).

The simplest form of an outlier is that which so often occurs in connection with a line of escarpment, the unequal recession of which has left a series of bays, promontories, and outliers. It is easy to see how the neck of a promontory may be gradually worn through by the action of rain. and springs until it is converted into an isolated hill, the very existence of such a hill becoming a proof of the former extension of the beds, and of the recession of the escarpment.

Very frequently the existence of the promontory and subsequent outlier is due to a slight synclinal curvature of the beds, the inward dip giving them a greater power of resistance to detritive agencies, and diminishing the liability to landslips. So often is this the case that the very occur