The microscope and its revelations . Microscopic Organisms in Levant Mud:—a, d, siliceous spiculesoiTethi/a; B, H, spicules of Geodia; c, Sponge-spicule (unknown); e,calcareous spicule of Ghantia; f, g, m, o, portions of calcareous skele-ton of Ecliinodermata; H, I, calcareous spicule of Gorgonia; k, l, n,siliceous spicules of Halichondria ; p, portion of prismatic layer ofshell of Pinna. COMPOSITION OF MARINE DEPOSITS. 745 tinually undergoing a slow but steady increase in thickness,tlie microscopic researches of Prof. Williamson* have shownthat not only are there multitudes of minute remains
The microscope and its revelations . Microscopic Organisms in Levant Mud:—a, d, siliceous spiculesoiTethi/a; B, H, spicules of Geodia; c, Sponge-spicule (unknown); e,calcareous spicule of Ghantia; f, g, m, o, portions of calcareous skele-ton of Ecliinodermata; H, I, calcareous spicule of Gorgonia; k, l, n,siliceous spicules of Halichondria ; p, portion of prismatic layer ofshell of Pinna. COMPOSITION OF MARINE DEPOSITS. 745 tinually undergoing a slow but steady increase in thickness,tlie microscopic researches of Prof. Williamson* have shownthat not only are there multitudes of minute remains of livingorganisms, both animal and vegetable, but that it is entirely oralmost whoUy composed of such remains. Among these wereabout 26 species of Diatomacege (siliceous), 8 species of Eora-minifera (calcareous), and a miscellaneous group of objects(Fig. 334), consisting of calcareous and siliceous spicules ofSponges and Gorgonise, and of fragments of the calcareousskeletons of Echinoderms and Mollusks. Fig. Microscopic Organisms in Chalk fromGravesend;—a, b, c, d, Textu-laria globulosa; e, e, e, Rotalia aspera; f, Textularia aeuleata;g, Planularia hexas; h, Navicula. 445. Now almost exactly the same collection of forms, withthe exception of the siliceous Diatomacese, is found in many * Memoirs of the Manchester Literary and Philosophical Society,vol. viii. 746 MICEOSCOPIC GEOLOGY. parts of the Calcaire Grossier of the Paris basin, as well asin other extensive deposits of the same early tertiary there is little doubt that a large proportion of the greatCretaceous (chalk) formation has a like composition; for manyparts of it consist in great part of the minuter kinds of Tora-minifera, whose shells are imbedded in a mass of apparentlyamorphous particles, many of which, nevertheless, presentindications of being the worn fragments of similar shells, orof larger calcareous organisms. In the Chalk of some locali-ties, Foraminifera constitute the principal part o
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