II.—On Faults in Strata

Abstract

A Little time back there appeared in the Magazine, some short papers on the subject of faults, and on the nature of the conditions and the forces through which these important structural features may-have been produced. The points I would now bring to notice are more elementary; they refer to the evidence for faults; hence involving the principal data upon which the higher discussion of the phenomena must be based, and the same data very argely affect our attempted restoration and history of bygone phases of the earth's surface. Faults and flexures in stratified rocks are the leading features through which we interpret the disturbances that have affected the earth's crust; and any looseness in determining their existence, form and amount, must vitiate many of our inferences. No one but an experimental field geologist can appreciate the difficulty of such determinations, and understand how faults are particularly liable to elude observation. This circumstance accounts for, but does not justify, the arbitrary use of faults in interpreting sections. To call in question the evidence upon such a familiar subject implies, of course, dissatisfaction at the manner in which it is handled in practice. This I at once admit, and will proceed to explain. The criticism I have to make is no more than might oceur to one who had never left his study; but I would state that with me it has had a most practical origin: in the progress of the work of the Geological Survey of India, several great boundary faults have been proposed in connection with our main rock-series, and in some cases published descriptions have been already given; but both on the score of the in-sufficiency of the evidence brought forward, and after personal examination in the field, I am unable to admit that some of the features in question can, without very implicit qualifications, be brought within the received definitions of a fault.