2 J. J. Harris Teall — On a Faulted Slate. 



■will be likely to ensue, great lateral pressure might thus be produced, 

 and any open cracks would become filled up. Such in brief is the 

 theory which Prof. Jukes applies to the trough faults of the South 

 Staffordshire Coal-field, and which Prof. Phillips had previously 

 applied to the Malvern area. 



In his work on the Geology of the Weald, Mr. Topley refers to 

 the above theory, and at the same time points out a different mode 

 of explanation, which is unquestionably true for certain faults to 

 which he especially directs attention. He shows that if a district 

 be faulted at two distinct periods, by faults which have the same 

 general strike, but which are inclined to each other, then troughing 

 must necessarily arise. This mode of explanation has one great 

 advantage over that adopted by Professors Phillips and Jukes. It 

 easily explains those cases in which the faults are clean cut ; that is, 

 in which there is no considei-able thickness of fault rock. If tho 

 faults producing the trough were simultaneous, then the angle of 

 the wedge would not usually correspond with that of the space into 

 which the wedge is supposed to slip, and clean cut faults would bo 

 the exception instead of the rule. Of course the real test of the 

 accuracy of Mr. Topley's mode of explanation is to examine the 

 point of intersection of the two faults, and trace the earlier fault on 

 the opposite side of the later one. This obviously is only possible 

 when the faults are small. 



Such being the two modes of explanation, it is interesting to 

 examine the slate in order to see what light it throws on the question. 

 A momentary glance is sufficient to show that Mr. Topley's mode 

 of explanation gives a perfect account of the appearances there seen. 

 Trace the faults of the A series in the top left-hand half of the 

 plate. They terminate abruptly at the line of the main fault. Other 

 and parallel faults occur on the opposite side, but they are not con- 

 tinuous across the slate ; the smaller faults of the B series shift 

 them again and again. The faults of the B series are therefore 

 later than those of the A series, and the troughing is the result of 

 the intersection of the two series. It is not necessary to suppose 

 that any long interval of time elapsed between the formation of the 

 two series. Indeed, I suspect that they are both connected with one 

 and the same set of earth-movements, though I do ;iot understand 

 how it is that they are produced successively and not simultaneously. 



It will be observed that the faults of this slate appear to be of 

 the ordinary and not of the reversed type. It must be remembered, 

 however, that we are ignorant of the position of the slate in relation 

 to the horizon. If the bedding were horizontal at the time of the 

 faulting, then the eifect of the faulting would be to extend the mass 

 in a horizontal direction ; but if, on the other hand, the bedding 

 were vertical, then the effect would be exactly the opposite. 

 Whether this particular mass of rock was subjected to horizontal 

 extension or to horizontal compression, or to some other kind of 

 deformation, cannot now be determined. It is interesting to note 

 that instances of what may be called reversed faults are common in 

 these slates. They are described by Mr. E. J. Hebert in the Geol. 



