Structural geology . by the discordance; it usually implies evidences of unconformity cited below are both physical andorganic. The secondary deformation of rocks with which thisbook is mainly concerned is only one of the factors to be consideredin unconformity. Stratigraphy, physiography and paleontologyare others,—in fact adequate understanding of the significance of 156 UNCONFORMITY 157 unconformity involves the widest range of geological subject is treated here principally in its relation to structuralgeology, and not in the broader sense that it is required for aphi
Structural geology . by the discordance; it usually implies evidences of unconformity cited below are both physical andorganic. The secondary deformation of rocks with which thisbook is mainly concerned is only one of the factors to be consideredin unconformity. Stratigraphy, physiography and paleontologyare others,—in fact adequate understanding of the significance of 156 UNCONFORMITY 157 unconformity involves the widest range of geological subject is treated here principally in its relation to structuralgeology, and not in the broader sense that it is required for aphilosophical understanding of its significance. Involving, as itdoes, considerations other than structural, it has been left to thelast chapter. IDENTIFICATION OF UNCONFORMITY Physical evidences of unconformity are: (1) Evidence of erosion, even without intervening deformationbetween formations. (2) Difference in Metamorphism:—-Stratigraphically lower rocksmay have suffered so much more metamorphism than overlying. Fig. 67. Ideal sketch to illustrate unconformities. After Spurr. A. Earlier lineof conformity; B. Later line. beds of similar lithology as to indicate the probability of a timeinterval between them. Original differences in lithology alsoinfluence the nature and extent of metamorphism. This factshould not be overlooked. (3) Difference in Deformation:—Stratigraphically underlyingrocks may be folded or cracked or may be schistose as result offlowage, while these features may be less conspicuous or lacking inupper beds of similar kinds, indicating a time interval betweentheir periods of formation. This criterion must be carefully used,for the differences in deformation may be due simply to varyingcompetence of the different beds. (4) Difference in Number of Igneous Intrusions:—Stratigraphi-cally underlying beds may be intruded by igneous rocks, whichhave not intruded the upper beds. This may not in itself beevidence of unconformity, but may confirm other evidences
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