feet wide at its base. The explanation then given was fully confirmed by these facts since observed and recorded by Mr. Johnson; and as it is applicable to all "Trough faults," and bears on the question of faults in general, it is now given here.
The simplest form of a Trough fault is that represented in Fig. 28, in which a portion of the bed A is dropped down between two
Fig. 28.
equal faults, C and D, which incline in such a manner that they must meet before reaching the lower bed B. It is clear that these faults, having an equal "throw" in opposite directions, must, when they meet, neutralize each other, and that no portion of the bed B can be "thrown" by them either upwards or downwards, or in any direction whatever. We may conceive it possible, however, that C may be continued as a fissure towards c or D similarly continued to d, though it seems much more likely that they should coalesce and continue in one intermediate fissure E.
The most obvious explanation of the cause of such a condition of things seems to be the following. Let a set of beds of rock of indefinite extent, among which are two marked ones. A and B, be acted on by an elevating force from below, causing them to bulge upwards as in Fig. 29. Mr. Hopkins has shown us that if this expansion be continued far enough, the result will be the production of one or more longitudinal fissures, commencing at some point below the surface. Let such a fissure be produced running from F to E, and at E let it split into two, E D and E C.
Fig. 29.
Now, if the expansive force be continued, and a consequent further elevation take place, it is clear that the beds will assume the further form shown in Fig. 30, the two side portions being still more bent upwards, and the fissure consequently gaping open, so
