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and gravels, beds of shells), and they may remain in this state for an indefinite period. Millions of years have elapsed since some of the early Tertiary strata gathered on the ocean floor, yet they are quite friable (e.g. the London Clay) and differ little from many recent accumulations. There are few exceptions to the rule that with increasing age sedimentary rocks become more and more indurated. Generally, the older the strata the more likely it is that they will

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