64 



lying beds seem almost to graduate into the coal measures. Several 

 details are given respecting ancient coal works, in which, in more 

 than one hundred places, the coal had been extracted by shafts sunk 

 through the magnesian limestone : and it is asserted that the quality of 

 the coal is never injured by the presence of the overlying formations. 

 Such injury is not only contrary to fact, but seems to be a physical 

 impossibility. 



§ 6. On the Faults affecting the Magnesian Limestone and Coal 

 strata, Trap dykes, fyc. — Examples are given of some great faults 

 which traverse both the carboniferous and the superior formations : 

 but it is remarked that many of the dislocations of the lower order of 

 rocks do not affect the upper. Respecting the age of the trap dykes 

 of the coal-fields, it is not possible to determine their epoch in com- 

 parison with that of the magnesian limestone, where they range up 

 to the escarpment : — and of such dykes there are only two examples ; 

 one of which does, and the other does not, pass through the beds of 

 the overlying series. 



Part II. — Internal Structure and great Subdivisions of the Magne- 

 sian Limestone. — Considered as a subordinate part of the new-red- 

 sandstone series, this formation admits of five natural subdivisions, 

 each of which is described in a separate section. 



§ 1. Lower Red-sandstone, or Rothe-todte-liegende. — In Yorkshire 

 this appears generally in the form of a coarse siliceous sandstone, of 

 a reddish tinge. It is associated with incoherent sand, red micaceous 

 shale, and sometimes with variegated marls. In Durham it is gene- 

 rally represented by a yellowish and nearly incoherent sand. In 

 some places it cannot be distinguished from the gritstone beds of the 

 coal measures : but as it commences in the edge of Derbyshire, and 

 is almost co-extensive with the magnesian limestone as far as the 

 mouth of the Tyne, it must on the whole be unconformable to the 

 inferior order, it is, however, of very unequal thickness, and its 

 upper beds are not always parallel to the strata of limestone which 

 rest upon it. In Durham, being of loose texture and pervious to 

 water, it throws the greatest difficulties in the way of mining opera- 

 tions carried on within the limits of the limestone. 



§ 2. (a). Variegated Marls, with irregular Beds of Compact and of 

 Shell Limestone. — This deposit is not either of great extent or thick- 

 ness, and is confined to a small part of the escarpment in Notting- 

 hamshire and Derbyshire. It is supposed to be contemporaneous 

 with the following subdivision : 



(b). Marl-slate, and Compact Limestone. — This is much more ex- 

 tensively developed than the preceding formation ; and though by 

 no means co-extensive with the yellow limestone, derives importance 

 from its constancy of position and from its fossils. Several localities 

 in the county of Durham are described ; and among the beds of marl- 

 slate of East Thickley, &c, two or three species of fern have been 

 discovered ; and seven or eight species of fish, four of which at least 

 seem to be identical with fish of the Copper-slate. 



§ 3. Great central deposit of Yellow Limestone. — It is subdivided 

 into the following modifications, each of which is described in detail. 



