MAGNESIAN LIMESTONE. 



160 



or France be derived from volcanic rocks? Were the theory of Von 

 Buch true, we ought to expect all limestone rocks in the immediate 

 vicinity of basalt to be magnesian ; but some experiments which I 

 made on the mountain limestone of Derbyshire, in near proximity to 

 the loadstone, proved that it did not contain so much magnesia, as 

 the beds that were much farther removed from the latter rock. 



The magnesian limestone is distinctly stratified ; the strata vary in 

 thickness from a few inches to several feet : in the northern counties 

 of England they are nearly horizontal ; they border the great coal 

 formation, and cover it on the eastern side. This formation of lime- 

 stone extends from the mouth of the Tyne to near Nottingham. The 

 colour of the limestone is generally a yellowish or reddish brown, va- 

 rying in intensity from a fawn color to that of an overburnt brick. 

 Some of the lowest beds are bluish and slaty, and intermixed with 

 marl ; but these beds seldom rise to the surface, and their nature is 

 litde known. Some beds of magnesian limestone have a granular 

 sandy structure, others are imperfectly crystalline : they possess a 

 considerable degree of hardness. A cellular variety of this lime- 

 stone occurs near Sunderland, which has received the name of Hon- 

 eycomb limestone : it agrees in most of its characters with the 

 rauch wacke of Thuringia, which is part of the zechstein formation. 



Many beds of magnesian limestone yield a foetid smell when rub- 

 bed. At Sunderland, the beds of magnesian limestone are more de- 

 veloped than in any other part of England that I am acquainted with. 

 In an account which I published of the Geology of Durham, in the 

 Philosophical Magazine for 1815,1 estimated the total thickness at 

 one hundred and fifty yards. This limestone has been bored into to 

 a considerable depth below the surface ; it was, as before mention- 

 ed, of a bluish colour. According to Mr. Farey, " under the yellow 

 beds of magnesian limestone, there are several beds of compact blue 

 limestone, abounding with Anomia (Terebratulse) and other shells ; 

 some of these beds differ entirely from the yellow and red beds, and 

 are more useful for agricultural purposes, particularly on the yellow 

 limestone lands."* This is the marl slate of Professor Sedgwick. 

 The lower beds of this formation are, I believe, more fully developed 

 in many parts of the Continent than in this country, which occasions 

 some uncertainty in classing them. The limestone of Thuringia, it 

 is agreed by the most respectable geologists, is zechstein, corres- 

 ponding with our magnesian limestone ; the lower part is a slaty 

 marl, sometimes impregnated with bitumen, and sometimes with 

 sand. This bed contains impressions of fish, like the lower beds of 

 the slaty Sunderland magnesian limestone : it contains also a small 

 quantity of copper pyrites, and the ores of lead, cobalt, zinc, bis- 

 muth, and arsenic, and is in some places worked. by the miners for 

 its mineral treasures. Above this bed there occurs a blackish-grey 



* Survey of Derbyshire, p. 157. 



