86 Rev. A. Sedgwick on the Geological Relations and 



III. Compact Magnesian Limestone. — Under this head are included true 

 compact dolomite, and many varieties of imperfectly compact magnesian lime- 

 stone. The former is found in Derbyshire and Nottinghamshire, in thin beds 

 associated with, and subordinate to, the crystalline varieties described above. 

 Its fracture is flat, conchoidal, and it is translucent at the edges; but in structure 

 it is very irregular, and often passes by insensible shades into other varieties. 

 A beautiful compact rock found in Huddlestone quarries is also a fine example 

 of this species. 



Of the imperfectly compact rock the examples are mucli more numerous, and are commonly 

 found in rather thin beds alternating with other more earthy masses in different parts of the for- 

 mation. Some fine thick strata of this variety (which are of grey colour, splintery fracture, and 

 translucent at the edges) are found on the banks of the Worksop canal, and have been exten- 

 sively used in forming the stone facings of the locks, and in other works of strong masonry. 



It may be convenient to bring under this variety the hard thin beds, occasionally of almost 

 porcellaneous texture, and often partially cellular, which in several places, especially in the 

 southern parts of Yorkshire, are found at the bottom of the formation, and are separated from 

 each other by thin bands of marl. Some of these local deposits are (as before stated) probably 

 contemporaneous with the "compact limestone and marl. slate" of the preceding section of this 

 paper. They agree with them in position, and resemble them in their mode of bedding, but differ 

 from them in colour and mineralogica! character. For examples of such deposits, I may refer 

 to the lowest beds of the yellow limestone near the village of North Anston, and to the system 

 of beds at the bottom of the outlier above the village of Keswick near the river Wharfe. 



To the preceding examples may also be added many of the harder beds of the formation, which 

 as they lose the compact character, pass at one extreme into an earthy texture, and at the other 

 into a granular dolomite. Black oxyd of iron, in the form of dendritic impressions, is constantly 

 found investing the exterior, or penetrating the substance of all the various rocks here described. 



IV. Laminated Structure. — Rocks of this structure (in which the laminse 

 are parallel to the planes of stratification, and look like successive layers of 

 deposit) are found abundantly in the county of Durham ; especially on the 

 coast, and in some quarries near Sunderland. They replace and pass into the 

 other modifications, but seem to be most developed in the middle and higher 

 parts of the formation. Their colours are dark brown, smoke-grey, and ash- 

 grey ; and they are sometimes extremely fetid. They are associated with 

 earthy and pulverulent beds or masses ; often exhibit subordinate regular 



lime, which, after being heated red, weighed 35.4 grains ; the remainder S^.SS was sulphate of 

 magnesia. Hence it appears, that the carbonate of lime and magnesia in this specimen are re- 

 spectively 26.2 and 22.75 grains. 



" Another portion of the same specimen weighing 50 grains being exposed to a white heat for 

 an hour in a platina crucible, lost 23.5 grains of carbonic acid. Now if it be supposed that this 

 •was divided equally between the lime and the magnesia, the result will be 26.85 carbonate of 

 lime, and 22.2 carbonate of magnesia. This accords so nearly with the analysis, that we may 

 conclude the supposition to be true, and that the specimen is a true dolomite." 



