(d) Extension or stretching of the rocks will have been frequently produced, causing rupture, and the resulting fissures are usually filled with mineral-veins, though this occurrence is by no means characteristic of rocks which have been affected by thrust-planes.
(e) Chemical changes may have occurred which have resulted in the reconstitution of some of the rock-constituents, which may crystallise where pressure is least, thus we often find rocks which have undergone movements of the type we are considering marked by the existence of sericitic films upon the surfaces.
Another reservation must be made when considering the law of superposition. The test is only applicable for limited areas. Suppose we find a deposit of clay a resting upon another deposit of limestone b in the south of England, and can prove that the apparent succession is the true one, that is, that there has been no inversion; it is clear that the test of superposition is applicable in that area. Now, we may be able to trace the two deposits continuously across the country, one as a clay, the other as a limestone; so that when we reach the north of England we find the clay a still reposing upon the limestone b. The test of superposition is applicable in that area also, the clay of the northern area being newer than the limestone of the same region. But, for reasons which will ultimately appear, it by no means follows that the clay of the north is newer than the limestone of the south, although the two deposits are continuously traceable with the same lithological characters; it may have been formed simultaneously with the limestone of the south, or even before it. Something more, therefore, than the test of superposition is necessary in order to make out the relative ages of continuous deposits in a wide region, and this is still truer in the case of deposits which are discontinuous, whether separated from one another by the sea, or by outcrops of older or newer rocks.
A few words of warning may be added with reference to the detection of bedding-planes. A bedding-plane is one which separates two beds, and its existence is determined during the deposition of the beds. Many other planes are formed in rocks subsequently to their deposition, and it is not always easy to distinguish these from true bedding-planes. That even experienced observers may be led astray is shown by the fact that, of recent years, it has been proved that great masses of rock have been claimed as of sedimentary origin, and their apparent order of succession noted, which are in truth naught but irregular masses of intrusive igneous rocks affected by divisional planes which simulate bedding, produced in the rocks subsequently to their consolidation. Joints, faults, and cleavage-planes may all at times simulate planes of bedding, and it is frequently very difficult to distinguish them in the limited exposures with which a geologist has oftentimes to deal. It is easier to make suggestions for distinguishing bedding-planes from other planes which simulate them, than to apply the suggestions in practice, and the student of field geology will find that experience is the only guide, though after years of experience he may be confronted with cases where the evidence is insufficient to convince him that he is dealing with planes of stratification and not with some other structure.
From what has been remarked, it will be inferred that the test of superposition though of prime importance to the geologist is frequently insufficient to enable him to ascertain the true order of succession of the strata, and he is compelled to supplement this test by some other. There are several useful physical tests which may frequently be applied. Thus, if a rock a contains fragments of another rock b, under such circumstances as to show that the fragments of b were included in a during its deposition, it is clear that b is older than a. Here again, it will be found from what appears in a later chapter that the student is confronted with difficulties when actually examining rocks, for fragmental rocks of cataclastic origin, where the fragments have been formed as the result of fracture produced by earth-movements subsequently to the deposition of the rock, simulate epiclastic rocks in which the fragments were introduced during the accumulation of the deposits to so surprising a degree as sometimes to baffle the most experienced observer. Not only are the fragments of these cataclastic rocks broken up, but they may be further rounded so as to imitate in a remarkable manner the water-worn pebbles of an epiclastic conglomerate. Again, an older series of rocks may have had structures impressed upon them as the result of changes subsequent to their formation, and before the formation of a newer set which the latter therefore do not exhibit. Jointing, cleavage, and various metamorphic phenomena may thus be exhibited by the older rocks, but great care is required in applying this test, especially with a limited thickness of rocks, as one set may not exhibit the structures not because they were not in existence when the structures were developed, but because their nature is such that they were incapable of receiving or retaining the structures. For instance a mass of grit which is older than a mass of clay-slate may not be cleaved, because, although subjected to the pressure which produced the cleavage, it was of a nature not adapted to the development of cleavage structure.
On the whole, application of tests dependent upon physical features of rocks, does not often supplement to any great extent the information supplied by ascertaining the order of superposition, and in all cases, where possible, every other kind of information should be supplemented, by that which is acquired after examination of the included organisms of the strata.