83 Rev. A. Sedgwick on the Geological Relations and 



of the western escarpment^ but is spread over more than nine-tenths of the 

 area included between the eastern and western boundary of the whole forma- 

 tion. The difficulty of describing it does not arise so much from its great ex- 

 tent as from its complexity of structure ; for it presents incomparably more 

 changes in its external character, and in the arrangement of its subordinate 

 parts, than any other secondary formation of the British series. A detailed 

 account of one portion of it might present very few analogies with the details 

 exhibited by another portion, even in a neighbouring district. All, therefore, 

 which I shall attempt in this place will be, to describe some of the most re- 

 markable varieties of rock subordinate to this great system of beds ; consider- 

 ing only in a general point of view their relations to each other, and to the 

 deposit of which they form a part. 



I. Arenaceous Dolomite. — This modification of the rock is of an open 

 arenaceous texture, being made up of a congeries of small irregular crystals, 

 the forms of which cannot always be determined ; but they may sometimes 

 be traced to the inverse rhomb. 



It forms a considerable portion of the deposit which rests immediately on the coal-measures to 

 the north-west of Nottingham, and alternates with other varieties of magnesian limestone in dif- 

 ferent parts of Nottinghamshire and Derbyshire. In these districts, its prevailing colours are 

 reddish or yellowish brown ; and it is deposited in thin beds, which are sometimes used for flag- 

 stones or coping-stones, but are never sufficiently coherent to make a good building stone. On 

 exposure to fire, it often passes into a bright brick-red colour; and in some rare instances, native 

 specimens may be found in the quarries of this colour*. 



To the same variety I would refer various irregular concretionary masses, which near Knares- 

 borough, and in many other parts of Yorkshire, are found subordinately to earthy and pulverulent 

 beds of magnesian limestone. I have not noticed any specimens in the county of Durham which 

 can be classed under this head. 



This modification (especially as it is found in Nottinghamshire) Is a true dolomite, with a great 

 excess of iron; and, notwithstanding its crystalline texture, is probably in a great measure of 

 mechanical origin. For it is thin-bedded, separated by thin bands of marl and siliceous sand ; 

 and in some places a coarse sand coated with green earth enters partially into its composition. 

 Lastly, in some quarries near Bilborough, the separating red marl and sand increase so much in 

 thickness, that the dolomite becomes subordinate to themf. In such instances the carbonate of 



'5 



* The specimens here described are not easily reduced to a calx. They are, however, asso. 

 ciatcd with beds which are burnt for lime : but it is never of a white colour, being stained with 

 various colours derived from the metallic oxyds of the rock. 



t The lozccr red sandstone is not found in the neighbouring districts as a distinct formation : 

 but the localities alluded to above, may be considered as exhibiting an alternation of this sand- 

 stone with the lowest beds of yellow limestone. I have only noticed two other decided examples 

 of a similar alternation ; one in the escarpment at Glapwell in Derbyshire, the other on the right 

 bank of the Nid immediately above Knaresborough, There are, however, many instances in 

 which the bottom beds of magnesian limestone contain a considerable portion of sand, apparently 

 derived mechanically from the strata on which they rest; and thin bands of variously coloured 

 marls are not unfrequently interposed between the beds in this part of the formation. 



