PRECIPITATION OF LIME IN THE SEA 331 

 CHEMICAL LIME AND MAGNESIA DEPOSITS. 



Chemical Precipitation of Carbonate of Lime and Mag- 

 nesia IN THE Ocean. In the modern oceans the cliemical precipi- 

 tation of Hme and magnesia as limestones and dolomites is almost 

 unknown. It is true that crystals of dolomite grow in vugs and 

 cavernous openings of coral reefs (Skeats-50), and calcite and 

 aragonite crystals were found abundantly in the cavities between the 

 organic fragments composing the coral rock in the deeper portions 

 of the Funafuti core, but those deposits belong rather to the cate- 

 gory of diagenetic modifications. This general absence in the 

 modern sea of such deposits is not a safe argument for the similar 

 freedom of the ancient sea from these deposits. Indeed, there are 

 many horizons where limestones and dolomites occur, which, from 

 the absence of organic remains, are difficult to explain as of other 

 than chemical origin. This is especially the case with the pre- 

 Cambric limestones, for which Daly (t2:7(5j) advocates a chemical 

 history. He suggests that the lime of the pre-Cambric ocean was 

 precipitated on the ocean floor by the agencies of decaying organic 

 matter. This accumulated in quantity owing to the absence of the 

 bottom scavengers which had not then come into existence, and 

 which now keep the sea bottom free from organic matter to a 

 large extent. Furthermore, the low temperatures of our sea bot- 

 toms permit only a very slow putrefaction of the organic matter 

 which does remain. In earlier seas, however, temperatures may 

 have been much higher, permitting such putrefaction on a large 

 scale. 



During putrefaction ammonium carbonate is given ofif in large 

 volumes. This converts the chloride and sulphate of calcium into 

 carbonate, which is then precipitated, the reactions being: 



CaSO, + (NHJXO, = CaC03 -f (NHJ.SO, 

 CaCl, + (NHJXO., = CaC03 -f 2NH,C1. 



Experiments (Irvine and Woodhead-30 :;^p. Quoted by Daly- 

 II : 1 01) have shown that sea water is readily modified by putre- 

 fying organic matter or effete substances derived from living ani- 

 mals. Four small crabs, weighing 90.72 grams, were placed in sea 

 water absolutely free from carbonate of lime. After twelve months 

 they produced an alkalinity in the water equal to the production of 

 45.36 grams of calcium carbonate. This was due to the decom- 

 position of calcium sulphate by the uric acid and other efifete 

 matter. In a second experiment, with temperatures ranging from 



