opposite sides of the fault bend in opposite directions, the flexures may cross each other, and thus the fault apparently disappear, and again set on again along the same line.
If now, recurring to the deal plank, we were to make a cross cut to the crack before mentioned, it is obvious that by applying the requisite force near the junction of the two, we might permanently elevate or depress the included corner of wood. We should then have a representation of two faults, each increasing towards their point of intersection. Or if we continue the crack to the edge of the plank, which we may look on as a great boundary fault, we may bend one side so as to make the dislocation gradually increase from its commencement up to its termination. Of the latter case we have examples in the Corbyn's Hall and Brockmoor faults, that gradually increase their "throw" as they approach the western boundary fault.
It follows from the above considerations, that if there be only one or only two lines of fracture they must gradually diminish and end somewhere, and that the dislocated ground must have at least one place of maximum disturbance towards which it bends down either in one or several directions.
To have any piece of ground altogether elevated or depressed with regard to the whole of that surrounding it, it is necessary that it be bounded by at least three rectilinear faults, or by such an arrangement of curved fractures as amount in effect to three or more rectilinear ones.
These statements will, of course, be to the geometrician exceedingly simple, and scarcely worth making. The considerations involved in them, however, are sometimes hardly sufficiently attended to in geological surveying. One often sees a mere straight line representing a fault on a geological map, and sometimes is tempted to wonder what becomes of it, or how any mass of beds can be elevated or depressed in consequence of a single crack running through them. That such single-lined fractures do occur, however, the beds on one side bending down, while those on the other have remained stationary, or are perhaps bulged upwards, is proved by the occurrence of the east and west faults, before described, traversing the South Staffordshire coal-field.
The above considerations have to do with the longitudinal extension only of faults, but their vertical extent is equally worthy of study. This, however, is so wide a field that we shall only venture on a small portion of it, the subject of "Trough faults," and refer the reader for other parts of it to the papers of Mr. Hopkins in the "Cambridge Philosophical Transactions," vol. 6[1], and in the "Philosophical Transactions of the Royal Society of London," vols. 133 and 134, 1842-3, as also to a paper by Mr. Darwin, in the "Transactions of the Geological Society," vol. 5, p. 601.[2]
Of "Trough faults" we have several excellent and well-explored examples in the South Staffordshire coal-field, both on a large and small scale. We will take for examination one example of
- ↑ William Hopkins, Esq., M.A., F.G.S., &c. 1837. Researches in Physical Geology, "Transactions of the Cambridge Philosophical Society", vol. 6, part 1, pp. 1-84 (Wikisource contributor note)
- ↑ Darwin, C. R. 1840. On the connexion of certain volcanic phenomena in South America; and on the formation of mountain chains and volcanos, as the effect of the same powers by which continents are elevated. [Read 7 March] Transactions of the Geological Society of London (Ser. 2) 5 (3): 601-631, pl. 49, 3 figs. (Wikisource contributor note)
