internal Structure of the Magnesian Limestone. 83 



lime and magnesia appears to have been mechanically deposited along with the other materials 

 which constitute the system of beds ; and if these earthy salts were derived from the destruction 

 of pre-existing dolomites, they would be supplied in that definite proportion which would enable 

 the comminuted particles to reunite and form a simple crystalline rock. 



II. Small-grained Dolomite. — This variety is also a crystalline dolomite ; 

 differing from the preceding- in being smaller-grained, and not having the 

 same open arenaceous texture. It exhibits various shades of colour from 

 ochre-yellow to yellowish white, and rarely becomes almost snow-white. 

 When mixed with impurities, or alternating with other modifications of the 

 magnesian limestone, it sometimes is brown, dull brick-red, or bright red. 

 The finest specimens are of a glimmering pearly lustre ; and a number of 

 minute black spots (occasionally aggregated in stellated forms) are often irre- 

 gularly diffused through them. Their fracture is generally uneven, and the 

 fragments into which they break are of irregular form. Beds, or crystalline 

 masses, answering more or less perfectly to the preceding description, are 

 found in various parts of Durham : for example, at Coniscliff Castle, Eden 

 Dean, Tunstall Hill, Black Rocks, and other places on the coast ; but in all 

 these places they are entirely subordinate to other varieties, and are neither 

 of sufficient regularity nor extent to be used in architecture. 



In the same subordinate form this dolomite occurs in various parts of York- 

 shire ; and in Nottinghamshire and Derbyshire it passes into, and alternates 

 with, the former modification. Its most perfect development may, however, 

 be seen in various ancient quarries, which, in the long range of the formation 

 from Nottingham to Bramham Moor, have been opened in the upper part of 

 the deposit I am attempting to describe. Any thing beyond a short notice of 

 these remarkable quarries would be incompatible with my present object. 



1. In the extensive quarries of yellow dolomite which are opened on the south side of Mans- 

 field, the lowest beds are thin, and are separated by thin layers containing greenish sand. Some 

 of these beds are arenaceous, and pass into the variety (No. I. p. 82.) above described; others 

 form an excellent building stone. The upper quarries are composed of irregular beds or tabular 

 masses, some of which are twelve or fourteen feet thick. They are partially tinged with green 

 sand and other impurities, and also contain a few veins and crystalline nodules of sulphate of 

 barytes ; but large blocks of nearly pure dolomite may be separated from them. The whole 

 system above described is surmounted by the lower marly beds of the forest sand. 



2, On the east side of the glen which descends to Mansfield, is a quarry which lays bare a 

 system of beds, about fifty feet thick, of very extraordinary character. The bottom beds are about 

 twenty in number, and vary from less than one foot to three or four feet in thickness ; but the 

 planes of separation are extremely irregular, and not continuous. They are of a dull red colour, 

 and might, without close examination, be mistaken for new red sandstone. The thin beds are 

 much used in building; and the thickest are hewn out into large troughs and cisterns, and in 

 that state are conveyed into all the neighbouring counties. Over the system just described is a 



m2 



