internal Structure of the Magnesian Limestone. 93 



beds, which have been extensively quarried*. In these beds are also fouud traces of organic 

 remains. 



Second modification, when the grains are of more regular form, and the 

 structure becomes ooHtic. Rocks of this structure are on the whole of rare 

 occurrence, though they abound in a few places. I never remarked them (at 

 least in any perfect form) in Nottinghamshire or Derbyshire : but in the 

 range of the formation in the southern parts of Yorkshire, especially between 

 the rivers Don and Went, there are several localities where they are well ex- 

 hibited. Some of these deserve notice. 



1. On the left bank of the Don near Cadeby, is a fine quarry exposing eight or ten hard sound 

 beds (some of which are nearly four feet thick), surmounted by eight feet of hard cellular amor, 

 phous masses of yellow limestone. The sound beds are partially oolitic, cut soft in the quarry, 

 harden by exposure, and then ring under the hammer, and externally resemble the Bath free, 

 stone. On examination, the grains are found to be less uniform in size, and not so perfectly 

 spherical as they are in the great oolite. Their surfaces, especially when seen through a lens, 

 have generally a glimmering lustre, and when broken they are frequently found empty. Some., 

 times they exhibit an aggregation in concentric laminae, in which case the exterior portion is 

 more crystalline than the interior ; and as these spherules are occasionally hollow, they have the 

 appearance of having been formed by an aggregation commencing at the surface, and proceeding 

 inwards. In some instances a fracture will pass through the centre of the oolitic grains, and in 

 that way expose on the surface a great number of minute spheroidal cells f. In other instances, 

 the separate grains resist the force of percussion, and the broken surface is studded (as in (he 

 great oolite) with an indefinite number of spherical particles. 



2. A few miles to the north of the preceding locality (for example, on the road side between 

 Marr and Hickleton) there are, in the very centre of the formation, several quarries exhibiting 

 beds similar in structure to those last described. These beds are, however, darker, coarser, and 

 less coherent ; and they contain many beautiful casts of a small turbinated shell. Moreover the 

 oolitic grains are less uniform in size ; and they occasionally pass into large pisolitic concretions, 

 which, on fracture, expose a number of concentric lamina. These laminae appear in some instances 

 to be hollow at the centre ; in other instances they have arranged themselves about the casts of 

 the small univalve above mentioned, and sometimes about a spherical congeries of minute oolitic 

 grains. 



3. At Stubbs Hill (on the road from Doncaster to Wakefield) are some coarse cellular red- 

 coloured beds, resting immediately upon the marls of the lower red sandstone. They contain 

 many organic remains, especially the casts of bivalve shells; and through them are disseminated 

 a number of oolitic grains of more uniform size and texture than in the preceding varieties J. 



* The quarries near the escarpment on both sides of the river Don produce some good build. 

 ing stone, wliich may be referred to this variety. Conisborough Castle is, if I remember right, 

 built of a stone which is allied to the modification here described. 



t One of the varieties above described, consisting of a congeries of hollow oolitic particles, in 

 some places (for example, in the great white quarries to the S.W. of Burton Leonard, near 

 Ripon) graduates into a rock of very singular appearance. In passing into a solid state, the 

 oolitic grains seem to have been completely blended ; and the rock has the character of an uniform 

 subcrystalline mass filled with minute spheroidal cells, like the vesicles of a piece of fine pumice. 



+ See Plate IV. Map 3. Sec. 3. 



