The microscope and its revelations . aria 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 ki
The microscope and its revelations . aria 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 of the minuteorganisms which can be recognized with the microscope(Figs. 335, 336); in other instances, the disintegrated prisms Fig. Microscopic Organisms in CTialk from Meudon; partly seen as opaque,and partly as transparent objects. of Finna (§ 336) or other large shells of the like structure (asInoceramus) constitute the great bulk; whilst in other cases,again, the chief part is made-up of the shells of Cytherina, a COMPOSITION or CHALK:—FLINTS. 74i7 marine form of Entomostracous Crustacean (§367). Differentspecimens of Chalk vary greatly in tlie proportion which thedistinctly-organic remains bear to the amorphous particles, andwhich the different kinds of the former bear to each other;and this is quite what might be anticipated, when we bear inmind the predominance of one or another tribe of animals orplants in the several parts of a large area. True Chalk seemsto differ from the Levant Mud, in the small proportion whichthe siliceous remains of Diatomacese bear, in the former, to thatwhich is mingled in the latter with the calcareous shells of Fora-muufera, &c.; and it s
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