REVIEWS. 
375 
Mr. J. D. Dan;i has pointed out a dolomite of recent origin in Matea, an 
elevated coral island, near Tahiti, where, among the limestones which he sup- 
poses to have been formed by the solidification of coral-mud, is one containing 
S';{ per cent, of magnesia, and another which, according to Prof. Silliman jun., 
yields liS'OZ per cent, of carbonate of magnesia. This dolomite is compact, 
llnelv graiudar, tenaceous, and at the same time cavernous ; its density in 
powder 2\S.'5, its hardness above i O. Mr. Hunt's analysis gave 38'25 per 
cent, of carbonate of magnesia 0'30 silica, 60'50 carbonate of lime. 
The preceding dolomites belong to marine formations, but dolomites are also 
said to occur in the lacustrine limestone at Diichingen, near Ulm, and iu the 
bro'wn-coal formation at Giessen. 
From the facts stated, it appears that the production of dolomites has been 
continued from the times of the earliest stratified rocks to the Tertiary period, 
and that it is even now going on. 
Apart from the altered crystalline dolomites of metamorphic strata, the 
generally crystalline texture of those of unaltered regions is remarkable. In 
some cases the rock is an aggregate of pearly, eleavable grains of dolomite, 
\\ hieh occasionally have but little coherence, or are in the form of loose sand. 
At other times the rock is concretionary, having an oolitic or a botryoidal 
structure, the masses often exhibiting a radiated arrangement ; more rarely 
compact varieties of dolomite are met with. The concretionary action has 
sometimes so far disturbed the original arrangement as to obliterate the nuxrks 
of stratification ; and most dolomites exliibit cavities which iiave often been 
subsequently filled with deposits of other minerals, and seem to indicate a con- 
traction, apparently attendant upon chemical change after the def)ositiou of the 
rock. 
A remarkable mode of occurrence is tliat iu which dolomite forms the 
cement of breccias and conglomerates. Rocks of this kind occur iu the 
Quebec division of the Hudsou River-group, where rounded fragments of 
limestone, shale, and even of d(ilomite, have oeeu re-cemented into a rock by 
the introduction of a crystalline ferriferous dolomite. Analagous to this is the 
well known conglomerate of the Permian formation near Bristol and iu otlier 
parts of England, where, iu hollows of the mountain-limestone are found ac- 
cumulations of fragments of this limestone, with others of coal-shale, mixed 
witli the bones and teetli of saui-ians, the whole cemented together by a red or 
yellow dolomite, and restmg unconformably upon the carboniferous strata. 
Similar conglomerates occur in the same formation in Normandy, where they 
inclose concretionary masses of nearly jiure dolomite ; while in the Permian 
rocks of the Vosges concretions of sandy dolomite occur, imbedded in layers of 
micaceous sandy clay, itself sometimes agglutinated by a dolomitic cement. 
A crystalline ferriferous dolomite tills the shells of Orthoceras, Pleitrofonian'a, 
and Mui-chisonia, as well as small fissures in the nou-magnesiau " Trenton" 
limestones of Ohawa ; and similar examples occur in the Chazy-limestones of 
Montreal. While these dolomitic casts thus occur in pure limestone, on the 
other hand beds of the Niagara formation, in some places, present purely cal- 
careous corals embedded in a yellow magnesian limestone. 
Mr. Hunt then gives us analyses and more minute descriptions of numerous 
limestones and dolomites from various parts ; the separation of the dolomitic 
portion from the limestone being effected on the principle laid down many years 
ago by Karsten, who pointed out that acetic acid in the cold scarcely attacks 
dolomite, although it readily dissolves carbonate of lime; hence, the magnesian 
limestones, when treated with this acid, leave a residue of dolomite. We have 
seen that pure dolomites consist of equal equivalents of the two carbonates : 
there are not wanting, however, rocks in which the magnesian carbonate pre- 
dominates over the lime, leading to the supposition of a mixture of magnesite 
