414 R. C. WALLACE 
The proportion of Mg salts in ordinary sea water is very small 
(o0.12-0.15 per cent Mg, increasing slightly with the depth). In 
inland seas, exposed to excessive ‘evaporation, it may rise to 4.15 
per cent Mg, as in the Dead Sea at 300 meters depth. Such 
exceptional conditions could not have prevailed; indications point 
to a clear, rather shallow sea, with recurrent periods of slight 
sedimentation. It may well be, however, as indicated by Steidt- 
mann,! that the seas of the early Palaeozoic contained a slightly 
greater proportion of Mg salts than do similarly situated seas 
of today. The proportion of MgCO, in the paler-colored lime- 
stones shown by the analyses represents the amount of MgCO, 
that was deposited under normal conditions in these seas. It 
may be taken as the solid phase in equilibrium with the Ca and 
Mg salts in solution at that particular temperature and pressure, 
though undoubtedly a small proportion of the MgCO, was originally 
introduced as an ingredient in the composition of the calcareous 
shells. No recrystallization, and consequently no dolomitization 
in the strict sense of the word, has taken place. Presumably the 
MgCoO, exists in solid solution with the very fine-grained calcitic 
material in which the broken shells are imbedded. 
It would appear, then, that the dolomitization (with recrystal- 
lization) of the darker areas was due to the presence locally of a 
larger percentage of Mg salts than the normal. From this point 
of view three suggestions as to the cause of the dolomitization might 
be examined: (1) that algae, either as attached fucoids or as unicel- 
lular algae of the plankton, had contributed the necessary salts; 
(2) that the markings are due to worm castings; (3) that sea 
water inclosed in cavities, such as the interiors of shells, had dolo- 
mitized the neighboring rock. 
The last of these suggestions we may consider first. Occasion- 
ally large shells are found in the center of the dolomitized areas, 
while no other trace of organic remains is to be found. If sea 
water replaced the softer parts of the organisms on their dissolu- 
tion, and was retained till the layers were buried under a gradually 
hardening ooze, the slight rise in temperature and pressure might 
be sufficient, where undisturbed contact between the solid and 
t Jour. Geol., XIX (1911), 323. 
