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Extract from beginning of chapter:


As previously mentioned, my frequent interviews with Becker over his collection of Washoe rocks impressed me with the close resemblance between rocks that he called pre-Tertiary and those he considered Tertiary, and led me to suspect the correctness of his distinctions and geological deductions. It seemed to me there were gradual transitions from one extreme of crystallization to the other, so I asked him to let me study the collection after he was through with it, which he willingly did. Arranging the rock specimens on a large table to conform to their position on his field map, it appeared at once that the boundary lines between what had been called different rocks did not separate specimens that were megascopically distinguishable from one another. Arranging the thin sections of rocks of various kinds according to the extent to which their groundmasses had crystallized, those with glass at one end of the series and the coarsest-grained ones at the other, it was found that there were transitions throughout each series and that the greater part of the so-called pre-Tertiary rocks had their exact equivalent among the Tertiary ones, some of the former being the coarsest-grained varieties, and some of the Tertiary ones being glassy. The age distinction had been based partly on the fact that some rocks were coarser grained than Tertiary rocks were supposed to be at that time and partly on the fact of more advanced alteration.

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