time, in consequence of these mechanical movements and of the chemical action thereby set up, the rocks have all been converted into varieties of gneiss and schist, and the foliation of the previously existing gneiss has been more or less obliterated by the secondary foliation which it has acquired in common with the other rocks. The igneous rocks too, which occur as intrusive masses and dykes in the stratified series, are in the central district converted into augen-schists, hornblende, and sericite-schists. Fig. 178 is an attempt to give a general idea of the structure of this region, but on such a small scale it is impossible to convey more than a feeble outline. Fig. 179 is a portion of it on a larger scale; and the reader should also refer to fig. 144, p. 454, to realize the kind of faulting.

The following extracts from the Report of the Geological Surveyors describe the extent to which the metamorphism of some of the rocks has been carried.

First, as to the gneiss, they note a gradual increase in the alteration of this rock, and in the obliteration of the first foliation as it is followed from west to east. Describing the belt of sheared gneiss and green schist which underlies the Moine thrust plane, they say: "The Archæan gneiss has been rolled out into a finely laminated slate, or slaty schist (mylonite), breaking into thin folia like leaves. of paper. All the various stages of deformation, from the crushed Archæan gneiss on the one hand, to the laminated slate on the other, can be clearly traced. The original constituents of the gneiss have been comminuted, but here and there broken fragments of the felspars occur, which are invariably drawn out in the direction of movement. . . . These finely laminated schists or slates show beautiful examples of fluxion structure, and their foliation-surfaces display closely set lines or striping,' indicating the direc tion of movement of the particles over each other, the general trend of the latter being E.S.E. Associated with these slates are certain belts of frilled' dark green schists, a detailed study of which points to the conclusion that they have been formed mainly out of dark hornblendic gneiss, the folia having been piled on each other by minute major and minor thrusts." (Op. cit., p. 430.)

The alteration of the conglomerate at the base of the

red sandstone forms another object lesson in metamorphism. "In its unaltered form this characteristic band of conglomerate is composed of more or less wellrounded pebbles of quartz-rock, gneiss, pegmatite, diorite, &c., embedded in a loose gritty matrix. But where it has been subjected to mechanical movement, the softer pebbles of gneiss and fragments of the basic dykes have been crushed, flattened, and elongated in the direction of movement. Indeed, in some cases they have been drawn out to such an extent as to form thin lenticular bands of micaceous or hornblende-schist flowing round the harder pebbles of quartz-rock. The latter still preserve their rounded form, but they are traversed by small step-faults,' tending to elongate them in the direction of movement. The original gritty matrix has been converted into a fine micaceous, or green chloritic schist, showing exquisite flow-structure winding round the elongated pebbles in wavy lines. In short, the matrix has been converted into a fine crystalline schist, and but for the presence of the deformed schistose pebbles it would probably be impossible to tell that the schist had a clastic origin." (Op. cit., p. 431).

In the upper stratified series the grits and quartzites are altered into quartz-schists, the shales are converted into dark micaceous schists, and the limestones into marbles. The dykes of felsite in these rocks are changed into a sort of soft greenish mica-schist, and the diorites into hornblendic augen-schist and gneiss.

In the more eastern districts there are great alternations of flaggy micaceous schist and of coarse micaceous and hornblendic gneiss, traversed in places by bands of pink and grey granite which have been converted into granitoid gneiss by mechanical movements.

CHAPTER XIV.

UNCONFORMITY AND Overlap.

'HAT dry land has always existed somewhere throughout all geological time we know from the very existence of mechanically-formed rocks, which are the memorials of its erosion and destruction. This axiom was well expressed by Jukes in the following passage: "Lyell long ago showed that the amount of such [detrition and] denudation is to be exactly measured by the quantity of the mechanically-formed aqueous rocks, and as our present lands show us vast sheets of sandstones and clays hundreds and thousands of feet in thickness and hundreds and thousands of square miles in extent, and as every particle of these enormous masses of rock is the result of the erosion of previously existing rocks, it follows that the amount of denudation must have been just as great as that of deposition. Just as when we see a large building we know that a hole or quarry must have been made somewhere in the earth, equal, at least, to the cubical contents of the solid parts of that building; so, where we see a vast mass of mechanically-formed aqueous rocks, we must feel assured that a gap was made somewhere in the surface of the earth equal to the solid contents of those rocks." Conversely, also, when we have ascertained that a large amount of rock material has been removed from any existing, or any ancient land surface, we can be sure that an extensive series of stratified deposits must have been formed somewhere out of the materials thus removed, and that their

'Jukes' "Manual of Geology," first edition, p. 280.

deposition went on pari passu with the detrition of the land surface.

The oscillations of level which have continually taken place have caused frequent changes in the form of the ancient continents, depressing some parts beneath the sea and elevating other parts, so as to raise portions of the adjoining sea-bottom into dry land. The tracts that formed dry land were constantly exposed to the detritive action of those agencies which were described in Part I., Chapters VI. and VII., and the tracts that sank below the sea were subjected to the planing and levelling action of the sea-waves (Chapter VIII.).

The result of both processes being to remove a great thickness of rock, and the special result of the latter process being to produce the more or less level surface which is termed a plain of marine denudation, it follows that when such surfaces are depressed far below the sea-level, and newer deposits are laid down upon them, there must be a break or discontinuity between the older rocks and the newer deposits. The older series may form a regular and continuous sequence of beds, but the process of deposition having been interrupted by a period of erosion and disturbance, there must naturally be a certain amount of discordance between the older and newer series. Such discordance constitutes unconformity.

The importance of these conclusions cannot be over-estimated, and without a knowledge of the facts and inferences to be described in the present chapter, the student could make but little progress in the study of stratigraphical geology.

Every country has its own geological sequence, comprising a great series of strata, which is divisible into many natural groups or stages, and between any two of these groups there are only two possible relations: they must be either in conformable or unconformable succession. These terms we proceed to define.

1. Conformable Succession.-Two sets of beds are said to be conformable when the base of the upper set rests evenly upon the surface of the lower set, that surface being everywhere one of original deposition. The plane of separation between two such groups simply marks an interval

of comparatively short duration, during which no deposition took place. Two or more conformable groups of beds will therefore form a regular sequence like that of the groups 1, 2, 3, 4, etc., in figs. 108 and 134.

2. Unconformable Succession.-Two sets of beds are said to be unconformable when the base of the upper set passes over and across the eroded edges of the lower set, the upper surface of the latter being in fact a surface of erosion. Such a plane of separation involves the lapse of an interval more or less long, during which parts of the beds previously formed were broken up and carried away, leaving a discontinuous sequence or a discontinuity in the stratigraphical succession, as in the lower part of fig. 172,

where the lower group of beds, m, m, have been tilted and planed off till the horizontal surface, A, B, was produced, on which the overlying strata 1 to 7 have been deposited.

This definition of unconformable succession is a very general one, and will include cases which are not generally called unconformities, though they are discontinuities. There are, therefore, two kinds of discontinuity or irregularity, either of which may cause a break in the regular upward succession of a series of stratified rocks; and, as the distinction between them involves important considerations, it is necessary that they should both be clearly defined.

A. Contemporaneous Erosion. This is a partial discordance caused by the erosion of channels and hollows in a lower group of beds before the deposition of the overlying