internal Structure of the Magnesian Limestone. 79 



these beds contain small crystals or crystalline nodules of galena ; and at Cullercoats blende 

 is associated in a similar manner with the beds of limestone*. 



Lastly, in the cliff between North.point (about two miles south of the Tyne) and Marsdeu 

 rocks, there is a fine exhibition of the inferior beds, in the form of a yellowish brown slaty lime- 

 stone, resembling the corresponding deposit at Pallion and West Bolden, surmounted by Jiffht- 

 coloured cellular and brecciated masses belonging to the superior formation f. Near Marsdeu 

 rocks we may also find many varieties of marl. slate and compact limestone : but in their arrano-e. 

 raent, and in their manner of association with the superior beds, they are extremely anomalous 

 and will be more properly noticed in a subsequent part of this paper. 



The appearances of the marl-slate and compact hmestone in Nottingham- 

 shire, Derbyshire, and Yorkshire, will require a very short notice. In the 

 two former counties thin-bedded varieties of magnesian limestone much coated 

 with dendritic impressions are found in some places immediately over the coal 

 formation. They occupy, therefore, the exact position of the marl-slate of the 

 county of Durham, and in some respects resemble it. In other respects they, 

 however, differ so much from it, and are so nearly connected with the upper 

 beds of magnesian limestone, that I have not ventured to arrange them in the 

 inferior group. The same remark may be applied to some nearly similar beds, 

 which in various parts of Yorkshire are interposed between the inferior red 

 sandstone and the great amorphous beds of yellow limestone. There are, 

 however, in some parts of that county, beautiful and unequivocal exhibitions 

 of the compact limestone and the marl-slate. 



1. In the lower part of the terrace which extends from Kippax towards Aberford, there 

 appears a thin-bedded blue compact limestone alternating with thin layers of marl, and resting 

 upon the lower red sandstone. It contains some obscure impressions and casts of bivalves, among 

 which is the genus Axinus of Sowerby. The lower beds are sandy, and might be mistaken for 

 coarse varieties of lias. The blue beds do not, I believe, contain magnesia; but they are (espe- 

 cially near the top of the group) associated with yellow beds containing that mineral. 



2. Beds of nearly the same character occupy the same geological position in the quarries 

 opened in the outlier (above described) north of Seacroft, and in various other places under the 

 magnesian escarpment. 



3. In the quarries at Linderick near Ripon, there is also a highly characteristic exhibition of 

 the compact beds. In these quarries which form the bottom of the terrace, the limestone is fetid, 

 nearly compact, thin-bedded, of a dark smoke-grey colour, and alternates with thin beds of marl. 

 The bottom of the formation is unfortunately not exposed. 



4. Lastly, the same group is seen in various quarries between Knaresborough and Ripon : for 

 example, at Yew Bank near Burton Leonard, in a perpendicular section nearly forty feet thick; 



* I am by no means prepared to deny that the metallic sulphurets may sometimes traverse the 

 beds above described in small strings or veins. I never, however, saw them in that form ; but 

 they always appeared to be imbedded in a way which seemed to indicate a contemporaneous 

 origin. 



t See Plate VII. fig. 4. 



