170 MAGNESIAN LIMESTONE. GYPSUM AND ROCK-SALT. 



compact limestone, very hard and tenacious, and distinctly stratified ; 

 over this is a cellular limestone ; and above this, a blackish brown 

 limestone, which yields a foetid smell when struck with a hammer, 

 and is in some places more than one hundred feet in thickness. All 

 these different beds Humboldt comprises under the name of zech- 

 stein, and agrees with other geologists in referring them to our mag- 

 nesian limestone ; the lowest bed rests on the red sandstone, and 

 sometimes alternates with it : but according to some geologists, the 

 connection between the two formations of red sandstone and zech- 

 stein is such, that the latter may be regarded as a subordinate forma- 

 tion to the former. The upper beds of what has been called zech- 

 stein alternate in Switzerland with beds of gypsum, which is inter- 

 mixed with rock-salt : some of the beds are argillaceous limestone, 

 containing ammonites and belemnites, and appeared to me to have a 

 greater resemblance to lias, than to magnesian limestone. 



In the lower part of the magnesian limestone in the West of Eng- 

 land, there is a conglomerate limestone, which contains fragments of 

 transition limestone, varying in size from several inches in diameter, 

 to very minute grains. 



The fossils in magnesian limestone are not numerous, at least in 

 the upper beds. Fossil fish have been found in some of the lower 

 beds in the county of Durham. One or two species of univalves, 

 and about nine species of bivalves, occur in this hmestone ; but these 

 shells are extremely rare, except in one or two situations. Some of 

 the shells, the productus and spirifer, nearly resemble those in the 

 mountain limestone, to which the magnesian limestone appears to 

 bear a greater analogy, than to any of the secondary strata above it. 



Magnesian limestone furnishes the most durable building stone that 

 is any where found in the upper secondary strata. 



I do not agree in opinion with those who regard the magnesian 

 limestone districts as unfertile; and perhaps no parts of England are 

 more salubrious, than those which have a subsoil of this limestone. 



A few small strings of lead ore have been found in the magnesian 

 limestone rocks near Sunderland. The limestone rocks on the coast 

 of Durham are wearing away by the violence of the ocean : they 

 have evidently extended much further to the east than at present. 



It has already been stated, that beside magnesian limestone, gyp- 

 sum and rock salt are associated with the new red marl and sand- 

 stone. Neidier of these minerals is however confined to this forma- 

 tion. Salt springs rise in many of the coal strata, and gypsum and 

 rock-salt are found both in the upper, secondary, and the tertiary 

 beds ; but the repositories of these minerals are more characteristic 

 of the new red sandstone, and may therefore, with propriety, be de- 

 scribed in the present chapter. 



Gypsum, both fibrous and massive, occurs in the new red marl 

 and sandstone : the fibrous gypsum forms numerous alternating 

 seams in cliffs of red marl: the seams vary in thickness, from one to 



