DOLOMITE 69 



DOG-TOOTH SPAR. Calcite, or carbonate of lime, crystallised in scaleno- 

 hedra. Such crystals are common in the Mountain-limestone of Derbyshire. 



DOG-WOOD. Comus sanguined, a small underwood known as the wild cornel, 

 and as the common Dogwood. Little splinters of this wood are used by the watch- 

 maker for cleaning out the pivot-holes of watches, and by the optician for cleaning 

 deeply-seated small lenses. Its peculiarity is that it is remarkably free from silex. 

 Toothpicks are also manufactured from dogwood. 



DOG-WOOD BARK. The bark of the Cornus jlorida, a plant much used in 

 the United States as a substitute for Peruvian bark in cases of fever. 



DOXiERXTX:. A variety of ' greenstone,' composed of augite and felspar. The 

 difficulty of discriminating these minerals gives the name, from Ao\fpJs, deceptive. 



DOxixr? ; DOXiX/Y-TUB. A mining term applied to a tub fitted with a perfo- 

 rated board, the dotty, to which a circular motion is given by a winch-handle, and 

 thus imparts a similar motion to the ore. See DRESSING OF ORES. 



DOLOMITE. (Dolomic, Fr. ; Dolomit and Bittcrspath, Ger.) A name applied to 

 Magnesian Limestone, in honour of a distinguished French geologist and mineralogist 

 of the last century, M. Deodatus Guy Silvanus Tancred de Gratet de Dolomieu. 

 This mineral is essentially a double carbonate of lime and magnesia, containing, in 

 the normal varieties : carbonate of lime, 54'35 ; carbonate of magnesia, 45'65 ; a 

 percentage composition corresponding to the formula CaO.C0 2 + MgO.C0 2 (CaWIg 

 2CO 3 ). The relative proportion of the two carbonates is, however, subject to con- 

 siderable variation, the species passing on the one hand into carbonate of lime, and 

 on the other into carbonate of magnesia. 



In its general characters, dolomite strongly resembles calcite or pure carbonate of 

 lime, from which, however, it may be easily distinguished by its greater hardness and 

 density, and by effervescing less briskly with acids. At the same time its crystalline 

 forms are neither so varied nor so complex as those of calcite. It is notable that the 

 crystals have a remarkable tendency to present curved faces, and from the nacreous 

 lustre which they often possess, are commonly termed pearl spar. The presence of 

 protoxide of iron replacing the isomorphous oxides, lime and magnesia, often causes 

 the mineral to assume a brown colour after exposure to the atmosphere, whence such 

 varieties are distinguished as brown spar. In this country, crystallised dolomite is 

 common in the lead -mines of Derbyshire, Alston Moor, Leadhills, and the Isle of Man : 

 the finest crystals, however, occur at Traversella, in Piedmont. 



The massive and finely-granular forms of dolomite are very widely distributed, 

 forming in certain districts rock-masses of considerable extent, and occasionally rising 

 into mountains of characteristic boldness, well seen in the Dolomites of the Tyrol. 

 In England, large deposits of magnesian limestone occur in the upper beds of the 

 Permian system, or that group of rocks which immediately overlies the coal-measures, 

 and of which the upper beds are commonly known as Zcchstein, a name borrowed 

 from their German equivalents. The principal deposits occur in the north-east of 

 England, extending from the mouth of the Tyne in a south-westerly direction through 

 Durham and Yorkshire, into the counties of Derby and Nottingham. In these beds 

 the dolomite frequently assumes singular concretionary forms ; whilst at Marsden, a 

 few miles south of the Tyne, it presents a schistose structure, separating into thin 

 layers of remarkable flexibility. A dolomitic limestone is also found in the so-called 

 Dolomitic Conglomerate of Bristol, a rock which occurs locally at the base of the Trias, 

 or New Red Sandstone, and contains fragments and pebbles of older rocks cemented 

 together by a limestone more or less dolomitic. In addition to the recognised forms 

 of dolomite, many limestone rocks, of various ages, are found, on chemical examina- 

 tion, to contain magnesia, and in some cases to pass into true dolomites. Dr. Sterry 

 Hunt has shown, moreover, that the veins in certain marbles are dolomitic, whilst the 

 base may be only an ordinary limestone ; and also that the substance of some fossil 

 shells is a dolomite, although the matrix is nothing more than limestone : this is 

 notably the case with some of the fossils of the Trenton Limestone (Lower Silurian) 

 of Ottawa, in Canada. 



The origin of dolomite has long been a standing enigma to the geologist ; but as the 

 question is one of only theoretical interest, its discussion would be out of place in this 

 .-irticle. 



As building stones, the dolomites have been highly valued, those varieties being 

 regarded as most durable which present a crystalline structure, and in which the 

 carbonates of lime and magnesia exist in nearly atomic proportions. The quarries in 

 the neighbourhood of Bolsover Moor, in Derbyshire, supplied the magnesian limestone 

 employed in the construction of the new Houses of Parliament. 



The dolomites of Durham are employed in the chemical works on the Tyne for the 

 manufacture of sulphate of magnesia, or Epsom salts, although at present to a lees 

 extent than formerly. 



