Note xi. § 42.
Fracture and dislocation of the strata.
182. The greatest part of the facts relative to the fracture and dislocation of the strata, belongs to the history of veins. The instances of slips, where no new mineral substance is introduced between the separated rocks, are what properly belong to this place. The frequency of these, and their great extent, are well known wherever mines have been wrought. In some of them no opening is left, but the slipped strata remain contiguous; in other cases, there is introduced an unconsolidated earth, often a clay, which may be supposed to have come from above, arid very probably to have been carried down by the water. In some such cases, however, there are not wanting appearances, which show the matter in the slip to have been forced up from below, as we find it to contain substances which could not have come from the surface.[91]
[91] Unconsolidated earth contained between the sides of a rock that has slipped, is frequent in Cornwall, and is called a Fleukan.
183. A very remarkable fact of this kind occurred not long ago, in digging the Huddersfield canal in Yorkshire; and a very distinct account of it is given in the Philosophical Transactions, by the engineer who directed the work. In carrying a tunnel into the heart of a hill, the miners came to what is called in the description a fault, throw, or break, or what we have here called a shift, which was filled with shale set on edge, mixed with softer earth, and in some places with small lumps of coal. The fault or space filled with these materials, was in general about four yards broad, and lay nearly in the direction of the tunnel, so that a considerable extent of it was visible. Beside the shale, it contained a rib of limestone, about four feet thick, which run parallel to the sides of the fault, and about four feet from the southern margin of it. On each side of this rib were found balls of limestone, promiscuously scattered, and of various sizes, from an ounce to one hundred pounds weight. The balls, when broken, were found to contain some pyrites near their edges; they were not perfectly globular, but flattened on the opposite sides, and similar to one another.[92] At the time when the account was written, about seventy yards of the rib had been discovered.
[92] Phil. Trans. 1796, p. 350.
184. Now, it is certain, that neither this rib of limestone, nor the balls that accompanied it, can have come from above, as there is no limestone within twenty miles of the place where they were found. They must, therefore, have been forced up from below, and no doubt belong to some limestone strata, which lie there at a great depth under the surface. The length of this fragment of rock, which, from the account, one must suppose to have been entire, conveys no mean idea, either of the intensity or regularity of the force by which it was brought into its present situation. In veins, it is not uncommon to meet with stones that appear to have come from a greater depth: but this is probably the most remarkable instance of the same phenomenon, which has appeared in a mere slip, and none, I think, can speak a language less liable to be misunderstood.
185. I shall here mention another mark of violent fracture, that has been observed in rocks of breccia or pudding-stone, which, though not of the same kind with the preceding, and of a nature quite peculiar, belongs rather to this place than any other. In rocks of the kind, just mentioned, it sometimes happens, that considerable portions are separated from one another, as if by a mathematical plane, which had cut right across all the quartzy pebbles in its way. None of the pebbles are drawn out of their sockets, that is, out of the cement that surrounds them, but are divided in two with a very smooth and even fracture. The pebbles, in the instances which I have seen, were of quartz, and other species of primary and much indurated rock.
Lord Webb Seymour and I observed pudding-stone rocks, exhibiting, instances of this singular kind of fracture, near Oban, in Argyleshire, about three years ago. The phenomenon was then entirely new to us both; but I have since met with an instance of the same kind in Saussure's last work. As the fact is of so particular a kind, I shall state it in his own words: The place was on the sea shore, near the little town of Alassio, between Nice and Genoa.
"En passant entre ces blocs de breche, j'admirai quelques-uns d'entr'eux, d'une grandeur considérable, et taillés en cubes, avec la plus parfaite régularité. Il y avoit ceci de remarquable, c'est que l'action de la pesanteur, qui avoit taillé ces cubes en rompant leurs couches, avoit coupé tous les cailloux des breches à fleur de la surface de la pierre, aussi nettement que si c'eût été une masse molle qu'on eût tranchée verticalement avec un rasoir. Cependant parmi ces cailloux, la plupart calcaires, il s'en trouvoit de très durs, de petrosilex, par exemple, même de jade, qui étoient tranchées tout aussi nettement que les autres."[93]
[93] Voyages aux Alpes, tom. iii. § 1731.
186. This description is no doubt accurate, though it involves in it something of theory, viz. that the fracture was made by the weight of the stone. This may indeed be true: the operation probably belongs altogether to the surface, and is one with which the powers of the mineral regions are not directly concerned. The phenomenon, however, appears to me, on every supposition, very difficult to explain. In the specimen which I brought from Oban, the smallest pieces of stone are cut in two, as well as the largest. The consolidation and hardness of the mass are very great, and the connection of the different fragments so perfect, that it is no wonder the whole should break as one stone. But still, that the fracture should be so exactly in one plane, and without any shattering, is not a little enigmatical; if it is indeed a fracture, it must be the consequence of an immense impulse, very suddenly communicated.