88 Rev. A. Sedgwick on the Geological Relations and 



accurate and systematic description ; but they may be conveniently separated 

 into the four following classes. 



1st. Where the lines of stratification are not obliterated, and where earthy and pulverulent 

 masses containing many cellular knotty protuberances are arranged in beds which are nearly 

 parallel to each other. Escarpments answering more or less perfectly to this character may be 

 seen in various parts of Yorkshire, especially in the neighbourhood of Pontefract and Ripon ; 

 and under such circumstances, the face of the rock sometimes resembles a gigantic work of ancient 

 masonry decomposing and crumbling into ruin. 



2nd. Where the earthy portions are rather more compacted than in the preceding class, and 

 where the harder portions are subordinate, and stand out in great cellular tuberous masses which 

 have no parallelism or regular arrangement. Examples of this structure may be seen iu many 

 parts of Yorkshire and Durham ; and a fine series of sections illustrating all its modifications are 

 exposed in the precipices on the left bank of the Nid near Knaresborough, 



3rd. In this class are included all those modifications of the rock where, on a great scale, the 

 earthy and pulverulent portions become subordinate, and where the knotty protuberances fre- 

 quently pass into each other. Portions of escarpments possessing this structure often admit of no 

 subdivision, and can be regarded as only one mass of irregular concretionary form. In some 

 places they resemble great irregular beds of brecciated structure thrown unconformably over the 

 stratified rocks which support them ; in others, they are rough and cellular, like a great mass of 

 scoria. Frequently they are of a more compact and continuous texture, but interrupted by 

 irre<fular cells, which vary from a fraction of an inch to two or three feet in diameter. The harder 

 portions of these masses are crystalline, compact, or earthy ; and generally of a yellowish brown 

 colour. The intervening spaces are often filled with a pulverulent magnesian earth of an ochre- 

 yellow, or pale-yellow colour. The smaller and more regular cells are (as in most other parts of 

 the formation) generally empty, and coated over with crystals of carbonate of lime. 



There is, however, no end to these modifications ; nor is it an easy task to convey a correct 

 notion of them by verbal description. They may be studied in the escarpments near the Mill or 

 river Nid below Knaresborough; at Clacks Heugh, and West Bolden on the Wear; in the 

 quarries which are opened in some of the round-topped hills near the western limits of the forma- 

 tion in the county of Durham ; in the clifi" near North Point in the same county, and in number, 

 less other localities. 



4th. In this class the irregular concretionary structure almost disappears, and the whole mass 

 passes into a nearly compact or porcellaneous structure, has a glimmering lustre, is translucent at 

 the edges, and has a fine-grained uneven fracture, here and there passing into splintery. The 

 rock is hard, occasionally giving fire with steel ; and is, in some places, brittle, flying under the 

 hammer into small irregular fragments. Having, however, no natural joints, it is not easily 

 broken into larger fragments. In other places it is tough, porcellaneous, and difficult of fracture, 

 and rarely fetid. 



It differs from the compact beds above described (No. III. p. 86.) — 1st, in having no marks 

 of stratification ; 2nd, in exhibiting on its weathered surface irregular nodosities, indicating a con- 

 cretionary texture ; 3rd, in its irregular fracture ; 4th, in containing many very minute cells or 

 vacuities which have no tendency to a spheroidal form ; and which, though frosted over with 

 minute crystalline points, are not regularly coated with crystallized carbonate of lime like the 

 cells in the more regular beds. 



Large irregular masses, answering more or less perfectly to the preceding description, lie 



