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[Patton introduces an ellipsis, apparently solely to mark the beginning of a new paragraph.] If the carbon dioxide dissolved in sea water decreases, some bicarbonate ions change to carbonate, thereby causing precipitation of calcium carbonate. Most are biogenic, and consist primarily of microscopic or macroscopic shells (or at least these make up a considerable fraction, and often almost all, of the rock volume).

The precipitate forms a fine, crystalline cloud which settles to the bottom. This is true of limestone production today too, most of which is in association with biological activity.

This report of fish material from Upper Cambrian rocks further extends the record of the vertebrates by approximately another 40 million years. Repetski, ``A Fish from the Upper Cambrian of North America,'' But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth?

It will be much more convenient to discuss this question in the chapter on the Imperfection of the geological record; and I will here only state that I believe the answer mainly lies in the record being incomparably less perfect than is generally supposed; the imperfection of the record being chiefly due to organic beings not inhabiting profound depths of the sea, and to their remains being embedded and preserved to a future age only in masses of sediment sufficiently thick and extensive to withstand an enormous amount of future degradation; and such fossiliferous masses can be accumulated only where much sediment is deposited on the shallow bed of the sea, whilst it slowly subsides.

The crust of the earth is a vast museum; but the natural collections have been made only at intervals of time immensely remote.

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