302 Geological Society. 



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 of the magnesian limestone, where they range up to the es- 

 carpment j 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 

 generally 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 : 



§ 2. (b). Marl-slate, and Compact Limestone. — This is much m e 

 extensively developed than the preceding formation ; and though by 

 no means coextensive with the yellow limestone, derives importance 

 from its constancy of position and from its fossils. Several locali- 

 ties 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. 

 ( 1 ) Dolomite, a simple crystalline rock. — (a) Arenaceous dolomite, 

 coarse, nearly incoherent, often in minute rhombs. — (b) Small- 

 grained dolomite. Many quarries of this variety are described as 

 existing on the back of the deposit, and extending from the neigh- 

 bourhood of Mansfield to Bramham Moor. The crystalline beds pass 

 into others of mechanical structure, and in some extreme cases con- 

 tain 20 or 30 per cent of siliceous sand. — (2) Compact magnesian 

 limestone. — (3) Laminated. — (4) Earthy. — (5) Masses of irregular 

 concretionary structure. — (C) Beds or concretionary masses of cry- 

 stalline limestone without magnesia. Examples of these are derived 

 from quarries near Ripon, Knaresborough, and Newton Kyme, &c. 



