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strata were well jointed, and at a time when the stratification planes were the leading lines of weakness. The dikes connected with the sills have irregular contact planes. The later dikes which cut the sills follow master joints. There is reason, in this field at least, for supposing that intrusion in the form of sheets tookdoi:10.1126/science.2.47.701 fatcat:nvl75djifzdknkuhjcm64gwrsq