Transactions of the American Institute of Mining Engineers Volume 9 - Softcover

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1881 Excerpt: ...employed by some members of the Geological Survey in the construction of many of the sections in Pennsylvania. But there is a serious objection to any system, involving in whole, or in part, the method of equal thicknesses, viz., the gradual obliteration of the surface flexures in the underlying rocks. This is illustrated by Fig. 6, which is constructed partly on this system. It will be observed that the steepest dips are along the line of observed exposures; above and beneath this line the axes rapidly fade away, and can hardly be seen in the basal stratum. The same result obtains in the (restored) formations lying above the present surface. Sections constructed on this principle (Figs. 4 and 6) will, therefore, always show the maximum curvatures along the line of observed dips. It is hardly necessary to say that any method uniformly giving these results must be essentially false. The apparent regularity of profiles so constructed lends false pretensions of accuracy to the section. Under one sequence of conditions only does this method give results approximating the true underground structure. Having reference to Fig. 6, let us suppose the lower strata to consist of hard, massive sandstones, or conglomerates, and the upper rocks of soft slates or shales. If a lateral compressing (flexing) force acts upon this series, the lower massive formation will yield in curves of large radius, while the overlying softer strata compressed in thetrough thus formed will be thrown into a series of crimples resulting in flexures similar to those shown in the section. If, on the contrary, the observed dips in the upper rocks are in hard strata underlaid by softer rocks, these latter would suffer the greatest contortion, whereas the construction shows them nearly flat. I hav...