Outlines of field-geology / by Professor Geikie.

  • Geikie, Archibald, 1835-1924.
Date:
1876
Catalogue details

Licence: Public Domain Mark

Credit: Outlines of field-geology / by Professor Geikie. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    coast-section, are apt to be but small and trifling. The larger faults—those -which powerfully influence the geological structure of a country—are seldom to be caught in any such visible form, blow why is this] Different reasons may be assigned, each of weight. First of all, it is evident that along lines of great dislocation there must have been, on the whole, greater pressure and a greater grinding and fracture of the fissure-walls than in clean, sharp cracks, where the rocks have been displaced only a few feet or yards. This broken rock on the line of fault crumbles down more than the solid rock on either side beyond it, or is more easily excavated and removed. So that whether on a cliff or on a flat surface, the actual fault is apt to be concealed by super- ficial detritus. Then again, large faults often bring together rocks of considerably different degrees of durability. The less-lasting material decomposes, and its debris goes to cover the actual junction-line between the two formations. Another reason may be sought in the extensive deposits of gravel, clay, or other superficial materials which are spread over the surface of a country and conceal the solid rocks. A line of fault is one of weakness, presenting facilities for attack by the denuding forces whereby it is hollowed out, so as to become a receptacle for these superficial deposits. In the consideration of faults, therefore, two questions obviously arise. How does a geologist recognise faults when he sees them 1 and bow does he prove their existence when he does not, and cannot, see them ? I need not enter into any detailed answer to the first of these questions. The inspection of the section of a fault in nature will tell more in a few minutes than I could in an hour, and the lesson so received will be better remembered. A fault is not usually vertical, but inclined at a high angle. The rocks are commonly somewhat shattered on either side, the central parts of the fracture being filled with the broken rubbish. The breadth of broken material may vary up to a mass of many yards. If, on the face of a cliff', two different sets of rocks are brought together against each other along a steep line of junction, where they are both jumbled and broken, that line will almost certainly be that of a fault. The inclination of the sides of the fault is termed its hade, and slopes away from the side which has been pushed up or in the direction of that which has gone down. This