108 Rev. A, Sedgwick on the Geological Relations and 



Feet. Inches. 



5. Yellow, rubbly limestone much mixed with earthy impurities, and con.l ^ 5 or 6 



taining a considerable portion of galena / 



6. Dark shale passing into limestone 1 G 



7. Yellow, magnesian limestone, with a few carbonaceous stains 3 



No. 5. of this section is unquestionably abed ; and the galena is not a watcrworn, mechanical 



deposit but is apparently contemporaneous. It is found in lumps of considerable size, and pene- 

 trates the hardest portions of the rubbly limestone. In the year 1823 some profit was made by 

 excavating it, but the works have been since deserted. Some of the beds have the character of 

 mountain limestone; and the resemblance is rendered more complete by the innumerable petri- 

 factions of the Producta calva which appear in the dark bluish limestone. In the same beds are 

 rarely found some traces of corals like those of Humbleton quarry near Sunderland. 



There is a third section still further on the dip of the beds, which exposes a higher portion of 

 this series. The whole face of the quarry is composed of thin very dark beds, penetrated by white 

 contemporaneous veins, and very nearly resembling transition limestone. In this, as well as in 

 the two preceding sections, all the dark compact beds are, I believe, without magnesia*. 



Some of the highest portions of the magnesian limestone in the county of Durham (for example, 

 the beds of Fulwell Hill, the beds on the coast south of Marsden Bay, and some of the beds 

 south of Black Rocks), seem to be very nearly related to the part of the series which I have 

 been describing. They are, however, so intimately blended with the other modifications of the 

 formation, that I found it impossible, in description, to separate them from the general mass of 

 the yellow limestone. 



Such aie the principal facts connected with the range and extent of this 

 deposit. In that unequivocal part of it which rests upon the lower red marl, 

 there are, in general, but rare traces of organic remains at Cold Hill to the east 

 of Aberford : in a few other places they are, however, abundant, though in 

 an obscure form. 



Its thickness is of course variable, and cannot in any case be easily ascer- 

 tained ; for, with the exception of the section below Knottinglcy (and even 

 there it is disguised by a dislocation), there is not a single spot which exposes 

 the junction of the top beds with the upper sandstone and marlf. Perhaps 

 eighty feet may be assumed as its maximum thickness in the southern parts of 

 Yorkshire. 



§ 6. Deposits immediately superior to the Magnesian Limestone. 



The deposits which succeed the magnesian limestone are of enormous 

 thickness and extent, and form the subsoil of what may be called the great 

 central plain of England. They are, as is well known, principally composed 



* Lime which is burnt from the best parts of these quarries may be spread over the land at the 

 rate of six chaldrons the acre ; but of the lime derived from the magnesian beds, not more than 

 two cluUdrons can be used with advantage. A larger quantity would produce sterility. 



t See Plate VII. fig. 6. 



