Structural geology . SouthRange, Baraboo district, Wisconsin, showing joints formed by the folding ofweak, thin beds interstratified with thick, strong beds. After open gashes or tension joints may be seen crossing the curved compres-sion joints in the softer layers. Whatever the cause, the upper layers tend to extend themselvesfarther than the lower layers by shearing, producing the sheetingplanes between them. The same structure has been referred alsoto tension due to cooling of the igneous rocks while still undersedimentary load, the sheets being approximately parallel to t
Structural geology . SouthRange, Baraboo district, Wisconsin, showing joints formed by the folding ofweak, thin beds interstratified with thick, strong beds. After open gashes or tension joints may be seen crossing the curved compres-sion joints in the softer layers. Whatever the cause, the upper layers tend to extend themselvesfarther than the lower layers by shearing, producing the sheetingplanes between them. The same structure has been referred alsoto tension due to cooling of the igneous rocks while still undersedimentary load, the sheets being approximately parallel to the 1 Dale, T. Nelson, The granites of Vermont: Bull. 404 U. S. G. S., 1909, pp. ; also Bulls. 354 and 484, U. S. Geol. Survey. COMPRESSION JOINTS 27 original contact surface of the intrusive. Bearing in mind theparallelism of the sheets to the present erosion surface, and theirdiminution in number below the surface, the explanation of tensionby cooling involves the assumption that the present erosion sur-. Fig. 12. Sheet structure in granite. After Dale. face is nearly the same as the original contact surface, which cer-tainly is not always true. The sheets are crossed by vertical joints which partly result fromtension due to gravity acting on the thin sheets. Some of them alsomay be compressive. By application of the principles of breakingunder rotational or shearing strain given above, it will appearthat a complementary set of compression fractures should be 28 STRUCTURAL GEOLOGY expected approximately at right angles to the sheeting quarries these vertical joints may be in one or more intersectingsets. They are characteristically intermittent, extending througha given set of sheets and offsetting in the sheets above and infrequently they are curved.
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