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ON THE DEPOSITION OF CHALK. 



been deposited from subterranean waters, at a high temperature.* 

 Calcareous or cretaceous matter is also ejected during aqueous vol- 

 canic eruptions. According to Ferrara, streams of liquid chalk, or 

 chalk in a state of mud, were ejected from the mud volcano of Mac- 

 aluba, in Sicily, in 1777, which, in a short space, formed a bed sev- 

 eral feet in thickness. Beds of limestone may have been formed by 

 similar calcareous eruptions, in which the lime might be sometimes 

 in solution, and sometimes mechanically suspended ; and the numer- 

 ous remains of testaceous animals in limestone appear to indicate, 

 that the calcareous solutions were favourable to the growth of ani- 

 mals, whose coverings contain so much calcareous matter. Nor is 

 it necessary to suppose, that these aqueous eruptions were always 

 sudden, and attended with violent convulsions, for when a passage 

 was once opened, they may have risen slowly, and have been diffused 

 in a tranquil state, and by gradual deposition, or condensation, may 

 have enveloped the most delicate animals or vegetables, without in- 

 juring their external form. — Second edition, 1815. 



If the geologist can admit such a condition of the ancient world 

 as above described, a condition which, on a smaller scale, might be 

 proved to have existed since the period of authentic history : if he 

 will further admit, that, before the formation of chalk, a great portion 

 of what is now England, and the northern Continent of Europe, 

 was covered by a deep ocean, interspersed with islands, and sur- 

 rounded by ancient continents, and this few modern geologists will 

 deny; then, if we allow submarine aqueous eruptions of calcareous 

 matter, and siliceous solutions from thermal waters, to have been 

 poured into this deep ancient ocean, we shall have all the circum- 

 stances required, to form thick beds of chalk, interspersed with nod- 

 ules of flint. In an experiment on clay formed into a stiff paste, by 

 admixture with a saturated solution of alum, it was found, on break- 

 ing the clay when dry, that alum was interspersed through the mass 

 in distinct crystals and concretions. In the same manner, we may 

 suppose that the silex in the siliceous solutions, spread through the 

 calcareous matter, would separate into distinct concretions, filling the, 

 cavities and pores or zoophytes — such as sponges and alcyonia, or 

 of shells deposited in the chalk. Every fact connected with the his- - 

 tory of chalk, proves that it was formed in a very tranquil sea, and 

 not by the drift or detritus of more ancient rocks. Mr. Mantell, 

 whose almost daily observations on the chalk formation scarcely suf- 

 fer an important fact to escape his notice, says, that, in the whole of 

 these immense beds that he has examined, the occurrence of a single 

 fragment or pebble of more ancient rocks in chalk is extremely rare ; 



* M. Brongniart, to whom I sent a copy of this work, of the edition of 1815, 

 subsequently admitted a similar formation of the siliceous beds and millstone in. 

 the Paris basin, namely, that they were deposited by thermal waters holding silex 

 in solution. 



