. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 322 POTSTONES AT HORSTEAD. [Ch. XVII. Southern Hemisphere likewise, as Captain Maury observes, in lat. 13° S., long. 16° E., for example, siliceous Diatomacese and sponge spicules are the predominant forms instead of calcareous Rhizopods. If it be asked how the Diatomacese above alluded to can obtain a constant supply of silex in solution, I may remind the reader of the decomposition of felspathic rocks mentioned above (p. 42) as a copi- ous source of that mineral. A
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 322 POTSTONES AT HORSTEAD. [Ch. XVII. Southern Hemisphere likewise, as Captain Maury observes, in lat. 13° S., long. 16° E., for example, siliceous Diatomacese and sponge spicules are the predominant forms instead of calcareous Rhizopods. If it be asked how the Diatomacese above alluded to can obtain a constant supply of silex in solution, I may remind the reader of the decomposition of felspathic rocks mentioned above (p. 42) as a copi- ous source of that mineral. Almost all the great rivers which flow into the ocean must contain some of it, and springs charged with silex in solution must rise up in many parts of the bed of the ocean as they do on dry land. Dr. Buckland endeavored formerly to account for the recurrence, at so many distinct levels, of beds of nodular or tabular flint in the chalk, by supposing the periodical accumulation of widely extended layers of mud, made up of calcareous and siliceous matter. When a stratum Ave or six feet or more in thickness had accumulated, its partial con- solidation took place, during which the heavier silex sank to the bot- tom, forming nodules, or, if it was in sufficient quantity, continuous layers.* But the thickness of the masses of chalk intervening be- tween some of the strata of flint has always made this hypothesis somewhat unsatisfactory, although such segregation of siliceous matter helps us to conceive how isolated and scattered flinty nodules may sometimes be formed in the midst of a calcareous matrix. To explain a regular succession of flinty layers, Ave must seek out some intermit- tent action, favoring alternately the deposition of calcareous and siliceous matter. Many centuries would probably be required for the growth of microscopic organisms sufficient in quantity to form abed of white chalk. From a drawing by Mrs. Gmin. View of a chalk-pit at Horstead, near Norwich, showing
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