REVIEWS. 



375 



Mr. J. D. Dana lias pointed out a dolomite of recent origin in Matea, an 

 elevated coral island, near Taldti, where, among tlie limestones which he sup- 

 poses to have been formed by the solidification of coral-mud, is one containing 

 8-3 per cent, of magnesia, and anothei' wliich, accorduio- to Prof. Silliman jun., 

 yields 38-07 per cent, of carbonate of magnesia. This dolomite is compact, 

 finely granular, tenaceous, and at the same time cavernous ; its density in 

 powder 2'83, 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 Dachingen, near Ulm, and in the 

 brown-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, cleavable grains of dolomite, 

 which occasionally have but bttle 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 tlie original arrangement as to obliterate the marks 

 of stratification ; and most dolomites exhi])it cavities wliich have often been 

 subsequently filled with deposits of other minerals, and seem to indicate a con- 

 traction, apparently attendant upon chemical change after the deposition of the 

 rock. 



A remarkable mode of occurrence is that in which dolomite forms the 

 cement of breccias and conglomerates. Rocks of this kind occnr in the 

 Quebec division of the Hudson Uiver-group, where rounded fragments of 

 limestone, shale, and even of dolomite, have been 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 in other 

 parts of England, where, in hoUows of the mountain-limestone are found ac- 

 cumulations of fragments of this limestone, with others of coal-shale, mixed 

 with the bones and teeth of saurians, the whole cemented together by a red or 

 yellow dolomite, and resting unconformably upon the carlDoniferous strata. 

 Similar conglomerates occur in the same formation in Normandy, where they 

 inclose concretionary masses of nearly pure dolomite ; while in the Permian 

 rocks of the Vosges concretions of sandy dolomite occur, imbedded in layers of 

 micaceous sandy clay, itseK sometimes agglutinated by a dolomitic cement. 



A crystallme ferriferous dolomite fills the shells of Orthoceras, Pleurotomaria, 

 and Miirchison.ia, as well as small fissures in the non-magnesiaii " 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, leadhig to the supposition of - a mixture of magnesite 



