III.—The Rhythmic Deposition of Flint

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Abstract
Mr. G. W. Bulman, in an essay on “Chalk Flints and the Age of the Earth”, states that Professor Owen regarded the layers of flint in chalk as “the remains of successive crops of sponges which grew again and again according to some periodic law”. Mr. Bulman then observes that the only periods affecting rock-formation are annual, and he suggests that sponge-growth might be rapid during the summer months and that free-swimming reproductive spores might be liberated towards winter, the old sponges then dying off. This implies that the water at the depths in which the sponges lived in our Cretaceous seas was responsive to climatic change, and that the sponges were equally responsive. The suggestion breaks down, however, on another ground, when its author points out that it demands the deposition of 3 or 4 feet of chalk in a year, so that the whole of the English strata above the Albian might have accumulated in about four centuries. Mr. Bulman thereupon remains hesitant and enquiring.

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