504 Sir W. Dawson — The Animal Nature of Eo%o'6n. 



in the lagoon of an ancient coral atoll, while he finds that coral 

 and coral sands of the same elevated reef contain very little 

 magnesia. He concludes that the introduction of magnesia into 

 the consolidating under-water coral sand or mud has apparently 

 taken place — " (1) In sea-water at the ordinary temperature; and 

 (2) without the agency of any other mineral water except that of 

 the ocean " ; but the sand and mud were those of a lagoon in which 

 the saline matter was in process of concentration by evaporation 

 under the solar heat. Element has more recently taken up this 

 fact in the way of experiment, and finds that, while in the case of 

 ordinary calcite this action is slow and imperfect, with the aragonite 

 which constitutes the calcareous framework of certain corals, and at 

 temperatures of 60° or over, it is very rapid and complete, producing 

 a mixture of calcium and magnesium carbonates, from which a pure 

 dolomite more or less mixed with calcite may subsequently result.^ 



I regard these observations as of the utmost importance in 

 reference to the relations of dolomite with fossiiiferous limestones, 

 and especially with those of the Grenville Series. The waters of 

 the Laurentian ocean must have been much richer in salts of 

 magnesium than those of the present seas, and the temperature was 

 probably higher, so that chemical changes now proceeding in limited 

 lagoons might have occurred over much larger areas. If at that 

 time there were, as in later periods, calcareous organisms composed 

 of aragonite, these may have been destroyed by conversion into 

 dolomite, while others more resisting were preserved, just as a 

 modern Polytrema or Balanus might remain, when a coral to which 

 it might be attached would be dolomitized. This would account 

 for the persistence of Eozoon and its fragments, when other 

 organisms may have perished, and also for the frequent filling of 

 the canals and tubnli with the magnesian carbonate. 



The question now arises as to the mineralization of Eozoon 

 with serpentine, and more rarely, especially in the case of its larger 

 and lower chambers, with pyroxene. Connected with this is the 

 alternation, as above described, of serpentinous and dolomitic layers 

 in the limestone, as if in successive times the conditions were alter- 

 nately favourable to the deposition of magnesium in the form of 

 carbonate and in that of silicate. 



We learn from the "Challenger" Eeports that under certain 

 circumstances the presence of organic matter in oceanic deposits 

 causes an alkaline condition, tending to the solution of silica and 

 the formation of silicates. We also learn that siliceous matter in 

 a state of fine division (e.g. volcanic dust) may afford material for 

 the production of hydrous silicates, either directly or indirectly 

 through the agency of organisms forming siliceous skeletons. The 

 " Challenger " Eeports also show that the silicates known under 

 the name of glauconite, and thus deposited, contain several bases to 

 some extent interchangeable. Of these the principal are aluminium, 

 potash, and iron, though magnesia is also present. Some older 



1 Bulletin' Geol. Soc. Belgium, toI. ix (1895, p. 3). Also notice in Geol. Mag., 

 July 1895, p. 329. 



