DOLOMIZATION, 207 
are chemical segregations and not detrital. This structure is sufficient evi- 
dence that the dolomite is not a simple sediment, such as most limestones 
are, but has crystallized through the influence of solutions. There are only 
two possible methods, broadly speaking, by which such solutions could 
have acted to bring about this result. First, the dolomite crystals may 
have been precipitated originally as such from sea water highly charged 
with salts of magnesium and calcium; and, second, the process may have 
been metasomatic, the solutions having acted subsequently to the deposition 
of the rock and thus produced the crystalline dolomite which now exists. 
The idea that dolomites are formed by direct chemical precipitation by 
sea water has been somewhat advocated,’ but is not largely held at the 
present day. Bischof” has shown that when a solution containing carbon- 
ates of lime and of magnesia becomes so highly saturated that precipitation 
begins, the lime carbonate will nearly all precipitate before the deposition 
of magnesia carbonate begins, owing to the difference in solubility between 
the two salts, and that there would thus result a lower layer of nearly pure 
carbonate of lime and an upper layer of nearly pure carbonate of magnesia. 
From the mingling of the two layers some small amount of dolomite might 
be formed, but no thick and continuous bed could thus be precipitated. 
In addition to this theoretical evidence against the direct chemical precipi- 
tation of dolomite is the practical evidence that no such precipitation is now 
known to be going on. 
The second possible way for the formation of dolomite, namely, by 
metasomatic interchange between the materials of an already deposited rock 
and other materials brought by permeating solutions, is the one which has 
seemed most probable to recent writers. This secondary origin is in many 
cases actually proved, and the rock from which dolomite is formed by 
alteration is always limestone. Dana® has described the interesting case of 
the elevated coral island of Metia, north of Tahiti. On analysis the white 
compact coral limestone of this island was found to be nearly a true dolo- 
mite. From the general character of this rock it appeared to have been 
deposited in the shallowimg lagoon of the small coral island, and to have 
been derived from ar abrasion of the coral. Deposits of mud are frequent 
17. Sterry Hunt, Chemical and Geological Essays, p. 92. 
* Chemical and Physical Geology, English translation, London, 1859, Vol. III, p. 170. 
’ Manual of Geology, 4th edition, New York and London, 1895, p. 133. 
