f 



H. B. Medlicott — On Faults in Strata. 345 



beyond their present boundary ; bnt this negative difficulty would 

 be readily disposed of by denudation. The demand for the direct 

 evidence saved the case from total and fatal misrepresentation ; 

 within a length of 200 miles one section of the contact was found, 

 and it showed the younger strata abutting into and against a serrated, 

 weathered surface of the older ; the supposition of faulting being thus 

 inadmissable. 



These two cases differ in every particular ; but they teach the 

 same lessons — that dips by themseh^es give no presumption what- 

 ever in favour of a fault ; and that the direct evidence of the actual 

 contact, if it can give but poor evidence for a fault, may, on the other 

 hand, give the most positive evidence against a fault. 



It is not unlikely that by a proper search I should find I have 

 been saying what has been better said before now ; and that much 

 of the information I require is already accessible. The opportunity 

 for such an enquiry is not, however, available to me at present ; and 

 it is, in any case, abundantly evident that the remarks need to be 

 brought forward. In the work I have already referred to, faults are 

 mentioned on almost every page ; their vertical throw ranges from 

 11,000 feet downwards; they would rank among the most remark- 

 able structural features of the country. Any one who may wish to 

 be reminded of the important function performed by faults in 

 geology, need only to examine the beautifully drawn sections ap- 

 pended to that volume : the dotted lines above the surface are not 

 drawn merely to indicate the equivalents of distant outcrops, but to 

 represent the amount of matter that has been removed by denuda- 

 tion. The approximate accuracy of this interpretation depends to a 

 great extent upon the evidence for these faults, more especially of 

 those great boundary faults the indirect proofs for which can seldom 

 attain the same certainty as in the case of faults within the limits 

 of one series of strata. Yet in only one instance in this work could 

 I find specific notice taken of the direct evidence, and that only 

 incidentally and in a less important case, where (p. 143) it is said 

 that the marked occurrence of sliclcensides seems to indicate a fault. 

 There are some instances given of faults coinciding with veins or 

 dykes ; but veins and dykes, without any attendant faulting, are of 

 common occurrence. There are also cases given of the fissure and 

 the displacement along it being exhibited in the face of cliffs ; but 

 even this is not explicit enough. Of the independent use of the 

 indirect method excellent examples are given on p. 88, in accounting 

 for the reappearance of the Bala limestone in stepped outcrops. 



Of the illegitimate evidence from dips, numerous instances occiu: : 

 thus on p. 59, it is said "The evidence of the fault is perfect, since 

 both formations yield a sufficiency of dips, and are run to strike 

 directly at each other." Again on p. 191, "The fault on the south 

 is certain, black shales and their grits being on the coast visibly 

 thrown against the altered strata." A similar argument is used on 

 p. 169. But it is where the rocks in contact are most widely dis- 

 tinct, that the highly unsatisfactory nature of such evidence becomes 

 fully apparent, as on p. 198, the Carboniferous Limestone is said to 



