Part I. Sect, iv. § 2.] DOLOMITIZATION. 305 
calcareous formations due to organic secretions are often weakly 
dolomitic at the time of their formation, and may have their 
proportion of magnesium carbonate increased by the action of 
permeating water, as is proved by the conversion into dolomite of 
shells and other organisms, consisting originally of calcite or arago- 
nite and forming portions of what was no doubt originally a limestone, 
though now a continuous mass of dolomite. This change may have 
sometimes consisted in the mere abstraction of carbonate of lime from 
a limestone already containing carbonate of magnesia, so as to leave 
the rock in the form of dolomite; or probably more usually in the 
action of the magnesium salts of sea-water, especially the chloride, 
upon organically formed limestone; or sometimes locally in the 
action of a solution of carbonate of magnesia in carbonated water 
upon limestone, either magnesian or non-magnesian. lie de Beau- 
mont calculated that on the assumption that one out of every two 
equivalents of carbonate of lime was replaced by carbonate of mag- 
nesia, the conversion of limestone into dolomite would be attended 
with a reduction of the volume of the mass to the extent of 12°1 per 
cent. It is certainly remarkable in this connection that large masses 
of dolomite which may be conceived to have once been limestone 
have the cavernous, fissured structure, which on this theory of their 
origin might have been looked for. | gees 
Dolomite has been produced both on a small and on a great scale. 
In the north of England-and elsewhere, the Carboniferous Limestone 
has been altered for a few feet or yards on either side of its joints 
into a dull yellow dolomite, locally termed “ dunstone.” Similar 
vertical zones of dolomite occur also in the Carboniferous Limestone 
of the South of Ireland, together with beds of magnesian limestone, 
interstratified with the ordinary limestone. Harkness pointed out 
that the vertical ribs occur where the rocks are much jointed, and 
the beds where they have few or no joints... No doubt mere 
percolating water has in these instances been the agent of change. 

On the other hand, there occur great regions of dolomite with 
a crystalline structure, which, like that of the Eastern Alps, has. 
by some writers been regarded as altered ordinary limestone. In all 
probability, however, these masses became dolomite at the beginning 
by the action of the magnesian salts of the concentrated waters of 
inland seas upon organic or inorganic calcareous deposits accumulated 
previous to the concentration, their metamorphism having consisted 
mainly in the subsequent generation of a crystalline structure 
analogous to that of the conversion of limestone into marble.” 
Conversion of Vegetable Substance into Coal.—Exposed to 
the atmosphere, dead vegetation is decomposed into humus, which 
1 Q. J. Geol. Soc. xv. p. 100. 
_ ? On dolomitization, see L. von Buch, in Leonhard’s Mineralog. Taschenbuch. 1824 ; 
Nauman’s Geognosie, i. p.763; Bischof’s Chemical Geology, iii.; Elie de Beaumont, Bull. 
Soc. Geol. viii. (1836), p. 174. Sorby, Brit. Assoc. Rep. 1856, part ii. p. 77, and Address 
Q. J. Geol. Soc. 1879. A full statement of the literature of this subject will be found ina 
suggestive memoir by C. Doelter and R. Hoernes, Jahrb. Geol. Reichsanstalt, xxv. 
x 
