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All gravity anomalies come from horizontal variations in density. If the earth materials were in layers of horizontally uniform density, there would be no gravity anomalies no matter what the vertical variation in density might be. If layers with different densities are disturbed, anomalies in mass concentration are produced. In Figure 1. let us assume that the layers, 1, 2, 3, and 4 have successively higher density values. d1, d2, d3 and d4. The fiat-lying strata at the margins of the figure will produce no gravity anomaly there because there is no horizontal variation. Over the central part of the figure, these layers are disturbed by a structural uplift which produces the density contrasts indicated by the various hachured areas. In the upper part of the figure, layer 2 is uplifted into the normal area for layer 1 and produces a density contrast d2d1 as indicated. Other density contrasts would be produced as indicated on the diagram. In this instance, where each layer is of successively greater density, all the density contrasts are positive and the sum of all of their effects will produce the positive anomaly indicated by the gravity profile.

From this example, it is evident that any disturbance of normally fiat-lying layers of different densities will produce some sort of density contrast which will cause a gravity anomaly. The magnitude and form of the gravity effect depend

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