equally, while molten matter and loose ejecta issued here and there in less or greater abundance along the chief lines of rock-disturbance.

Similar geological structures on a smaller scale may be seen nearer home, and are well exemplified in the region of the Vosges and the Black Forest. These opposing mountains are the counterparts of each other, being built up of the same rocks, arranged in very much the same way. The basement rocks are granite and crystalline schistose rocks, which are overlaid by a series of Mesozoic strata. In the Vosges the dip of these strata is westerly, while the corresponding rocks in the Black Forest are inclined. towards the east. Between the two ranges, as everyone knows, lies the basin of the upper Rhine, a basin which, like that of the Jordan, has been determined by a number of parallel normal faults. The Mesozoic strata in the region surrounding the two ranges attain a thickness of at least 5000 feet, and there can be no doubt that these originally extended from west to east across what is now the basin of the Rhine. This is shown by the simple fact that the strata in question occupy that basin. (See Fig. 66, p. 164.) Doubtless the Mesozoic rocks were originally deposited in approximately horizontal positions. Subsequently the sea retreated from the area, and a wide land-surfaceprobably an elevated plain or plateau-occupied its place. Eventually, in early Tertiary times, the region was subjected to crustal movements, and traversed from south to north by a series of dislocations, with

FIG. 66. SECTION ACROSS THE Vosges and THE BLACK FOREST. (After Penck.) 1, gneiss and granite; 2, Bunter sandstone; 3, Muschelkalk; 4, Keuper; 5, Lias; 6, Dogger, or Oolite; 7, White Jura;

8, Tertiary; 9, Pleistocene; (2 to 7 Mesozoic

strata).

'downthrows in opposite directions. As a result of these displacements the Rhenish. basin came into existence, while the rock-masses along its margins were pushed up to form the ranges of the Vosges and the Black Forest. The crustal movements referred to appear to have been continued down to post-Tertiary times, and have probably not yet ceased, the frequent earthquakes experienced in the neighbourhood of Darmstadt being perhaps an indication of progressive

subsidence along lines of dislocation. It is interesting to note that these crustal movements have been accompanied from time to time by volcanic action. The well-known Kaiserstuhl near Freiburg, for example, is the skeleton of what must have been a very considerable volcano.

The evidence that subsid

ence in the Rhenish basin has continued into the post-Tertiary period is so striking that it may be briefly referred to here. Deep borings have shown that the Pleistocene deposits in the valley of the Rhine in Hesse occupy a profound hollow, surrounded on all sides by older rocks, the bottom of the basin being 270 feet deeper than the lowest part of its rim. at Bingen. These deposits, however, are not lacustrine, but fluviatile. Hence we must infer that fluviatile deposition has kept pace with the crustal movement. As the bottom of the Rhine valley has slowly subsided, the river has flowed on without interruption, continuously filling up the gradually deepening basin with its sediment. This is only another example of the fact that movements of the crust, whether of elevation or depression, have often proceeded so slowly that they have been unable to modify the direction of streams and rivers.

While we recognise the influence of earth-movements in determining the form of the surface in the region under review, it is obvious that much rockmaterial has been removed. The presence of the Mesozoic strata in the basin of the Rhine shows that these must formerly have extended continuously over the adjacent tracts. Yet they have since been largely denuded away from the higher parts of the Vosges and the Black Forest, so that the underlying crystalline rocks have been laid bare, and now appear at the surface over considerable areas.

When we turn our attention to regions of highly

dislocated rocks, where the crustal displacements are of much greater antiquity than those we have just been considering, the surface-features, we find, have often been so modified by denudation that the position and even the very existence of normal faults can be determined only by close observation. In other cases, however, they give rise to marked features at the surface.

The following section across a portion of the Lanarkshire coal-field is drawn upon a true scale. The section traverses several normal faults, the largest

FIG. 67. SECTION OF COAL-MEASURES (ON A TRUE SCALE) NEAR
CAMBUSNETHAN, LANARKSHIRE.

being a displacement of 350 feet, yet there is no feature at the surface to indicate its presence.

It is only by studying the geological structure that the existence of such dislocations can be discovered. The strata of the region in question are of much the same consistency throughout, and have therefore yielded equally to the various agents of erosion. Thus all inequalities of surface which may originally have resulted from faulting have been smoothed out. It is doubtful, however, whether such relatively small faults ever did show at the surface. The amount of displacement effected by them usually diminishes upwards, so that the highest coal-seams are hardly dis

located to such an extent as those which occur at lower levels. Many small faults, indeed, die out upwards altogether. And when we remember that the rocks now exposed at the surface were formerly covered by enormous sheets of strata which have since been removed by denudation, it is not hard to believe that even some of the larger faults of our coal-fields may actually have died out before the original surface of the Carboniferous strata was reached.

Some normal faults, however, are so very extensive the amount of displacement is so very greatthat we must believe they did reach the earth's surface at the time of their formation. Yet where these faults traverse strata having much the same character, they produce no inequalities of level at the surface. A good example is the Tynedale fault of the Newcastle coal-field, which has a downthrow in some places of 1200 feet, and yet its existence is not betrayed by the configuration of the ground. (See Fig. 68, p. 168.)

Great normal faults, however, usually do show more or less conspicuously at the surface. This is due to the fact that by their means areas of soft and hard rock are often brought into juxtaposition. Many examples might be cited from Great Britain. Thus in Scotland the Central Lowlands, consisting largely of relatively soft rocks, have been brought against the harder rocks of the Highlands on the one hand, and those of the Southern Uplands on the other. A