50. Dislocations or Faults.—When strata, once continuous, have been broken across, and displaced or shifted along the line of breakage, they are said to be faulted, the fissure along which the displacement occurs being termed a fault or dislocation. The simplest form of a fault is that shewn in the following diagram, where strata of sandstone and shale, with a coal-seam, S, have been shifted along the line f. The direction in which the fault is inclined[D] is its hade, and the degree of vertical displacement of the beds is the amount of the dislocation. Generally, the beds seem to be pulled down in the direction of the downthrow, and drawn up on the opposite side of the fault, as shewn in the diagram. Sometimes the rocks on each side of a fault are smoothed and polished, and covered with long scratches, as if the two sides of the fissure had been rubbed together. This is the appearance called slickensides. Slickensides, however, may occur on the walls of a fissure which is not a displacement, but a mere joint or crack. A dislocation is spoken of as a downthrow or an upcast, according to the direction in which it is approached. Thus, a miner working along the coal-seam S, from a to b, would describe the fault, f, as an upcast, since he would have to mine to a higher level to catch his coal again. But, had he approached the fault from c to d, he would then have termed it a downthrow, because he would see from the hade of the fault that his coal-seam must be sought for at a lower level. Faults are of all sizes, from a foot or two up to vertical displacements of thousands of feet. Powerful dislocations can often be followed for many miles across a country, running in a more or less linear direction. Thus, one large fault has been traced across the breadth of Scotland, from near St Abb's Head, in the east, to the coast of Wigtown, in the west. Every large throw is accompanied by a number of smaller ones—some of which run parallel to the main fault, while many others seem to run out from this at various angles. Faults are of all geological ages. Some date back to a most remote antiquity, others are of quite recent origin; and no doubt faults are occurring even now. In the following diagram, the strata, a, a, have been faulted and planed away before the strata, b, were deposited. Hence, in this case, it is evident that if we know the geological age of the beds, a and b, we can have an approximation to the age of the fault. If the beds, a, be Carboniferous, and those at b Permian, then we should say the fault was post-Carboniferous or pre-Permian.
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| Fig. 18.—Ground-plan of Large Main Fault and Minor Displacement Fissures. |
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| Fig. 19.—Faulted Strata covered by undisturbed Strata. |
51. Metamorphic and Igneous Rocks—mode of their occurrence.—In the foregoing remarks on the structure and arrangement of rocks we have had reference chiefly to the aqueous strata—that is to say, the mechanically, chemically, and organically formed rocks. We were necessarily compelled, however, to make some reference to, and to give some description of, certain structures and arrangements which are not peculiar to aqueous strata, but characterise many metamorphic and igneous rocks as well. To avoid repetition it was also necessary, while treating of joints, &c., to give some account of certain structures which are the result of metamorphic action. But, for sake of clearness, we have reserved special account of the structure and mode of occurrence of metamorphic and igneous rocks to this place. After what has been said as to the structure and arrangement of aqueous strata, it is hardly needful to say much about the crystalline schists. These the student will understand to be merely highly altered aqueous rocks,[E] in which the marks of their origin are still more or less distinctly traceable. As a rule, metamorphic strata are contorted, twisted, and crumpled, although here and there comparatively horizontal stretches of altered rocks may be observed. The regions in which they occur are often hilly and mountainous, but this is by no means invariably the case. The greater part of the mountainous regions of the British Islands is occupied by rocks which are more or less altered; the more crystalline rocks, such as mica-schist, gneiss, &c., being abundantly developed in the Scottish Highlands, and in the north and west of Ireland; while those which are less altered cover large areas in the south of Scotland, and in Wales and the north-west of England. Throughout these wide areas the rocks generally dip at high angles, and contortion and crumpling are of common occurrence. The finer-grained clay-rocks also exhibit fine cleavage planes, and are in some places quarried for roofing-slates—the Welsh quarries being the most famous. Here and there, bedding is entirely effaced, and the resulting rock is quite amorphous, and, becoming gradually more and more crystalline, passes at last into a rock which in many cases is true granite. The original strata have disappeared, and granite occupies their place, in such a way as to lead to the inference that the granite is merely the aqueous strata which have been fused up, as it were, in situ. At other times the granite would appear to have been erupted amongst the aqueous strata, for these are highly confused, and baked, as it were, at their junction with the granite, from which, also, long veins are seen protruding into the surrounding beds. Metamorphic granite, then, graduates, as a rule, almost imperceptibly into rocks which are clearly of aqueous origin; while on the contrary the junction-line between igneous granite and the surrounding rocks is always well marked. The origin of granite, however, is a difficult question, and one which has given rise to much discussion. Some further remarks upon the subject will be found in the sequel under the heading of Metamorphism.