Structural geology . urrent action of gravity, and toother causes. Whether a given fault is really tensional or compressional whenconsidered in three dimensions, whether it is subsidiary to a majorfault of different displacement, has been satisfactorily determinedin comparatively few instances. While the terms tension andcompression are freely applied to faults, this is really done on theunreliable assumption that the apparent displacement in a verticalplane represents the actual displacement. Means of identifying tension joints discussed on pp. 22-23 maybe used also for determining local tens
Structural geology . urrent action of gravity, and toother causes. Whether a given fault is really tensional or compressional whenconsidered in three dimensions, whether it is subsidiary to a majorfault of different displacement, has been satisfactorily determinedin comparatively few instances. While the terms tension andcompression are freely applied to faults, this is really done on theunreliable assumption that the apparent displacement in a verticalplane represents the actual displacement. Means of identifying tension joints discussed on pp. 22-23 maybe used also for determining local tension faults. 42 STRUCTURAL GEOLOGY Normal Faults Associated with Igneous Rocks:—Faults are likelyto be numerous within and adjacent to areas of igneous are especially numerous in surface volcanics. Such faults aremore or less irregular and discontinuous, and offset along crossfaults and joints, breaking the rocks into heterogeneous polygonalblocks. Displacements are both horizontal and vertical. Normal. Fig. 25. Block dislocated by movement between heave and upthrust, showingapparent normal faulting. After Ransome. faults predominate. Hinge faults are not uncommon. Thesefaults are well illustrated on many maps of western mining districtsprepared by the U. S. Geological Survey, notably those of the Ton-opah,1 Goldfield,2 Bullfrog,3 and Clifton 4 districts. 1 Spurr, J. E., Geology of the Tonopah Mining Distriet, Nevada: Prof. PaperNo. 42, U. S. Geol. Survey, 19(15. 2 Ransome, F. L., Geology and ore deposits of Goldfield, Nevada: Prof. PaperNo. 66, U. S. Geol. Survey, 1909. 3 Ransome, F. L., Emmons, W. H., and Garrey, G. H., Geology and ore depositsof the Bullfrog distriet, Nevada: Bull. 407, U. S. Geol. Survey, 1910. Lindgren, VValdemar, Copper deposits of the Clifton-Morenci district, Arizona:Prof. Paper No. 43, U. S. Geol. Survey, 1005. NORMAL FAULTS 43 It has long been suspected that there is some genetic connectionbetween faulting and igneous activity. Spurr ex
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