ALUM SHALE. 141 



No. 12 of the Boulby series. This is the lowest part of our strata, 

 appearing in the sea cliffs ; for the hard blue shale, forming the scar 

 at Boulby, is not a new member of the series, but the inferior part of 

 this ; the shale being hardest in the lowest portions of the bed, espe- 

 cially where the sea water prevents or retards its decomposition. 

 Like the main bed of aluminous schistus, this lowest sha!le is of great 

 thickness. At Boulby it rises more than lOO feet above the level of 

 the sea, and in some parts of Huntcliff, about 130 feet. Its depth 

 below the sea is yet unexplored ; for though borings for coal have 

 been tried along our northern shores, in various spots, near Skinnin- 

 grave, Saltburn, and Coatham, they have never been carried to such 

 a depth as to penetrate beyond the blue shale. The boring at Coatham, 

 conducted under the direction of Sir Charles Turner in 1788, reached 

 the depth of between 40 and 50 fathoms. This may be considered 

 as the lowest point in the aluminous strata hitherto attained ; for the 

 rocks opposite Redcar and Coatham belong to a lower portion of these 

 strata than the foot of Huntcliff. 



The lowest shale is of various qualities", but in general it is more 

 OT less sandy and micaceous. It is only imperfectly schistose^ for it 

 does not, like the principal bed, split into thin laminae, smooth and 

 fine grained; but has a coarse, earthy, dull aspect, and splits, or 

 rather breaks, into rough irregular pieces. Though Ave speak of this 

 shale as one bed, it is subdivided, especially towards the lower part 

 of it, into a number of successive beds, or cotirses; each of which 

 has usually a kind of hardened crust on its surface, while the rest of 

 the bed is more soft and friable. These crusts, generally speaking, 

 may be considered as various modifications of the shale itself, rather 

 than bands of other kinds of rock. In some of them, the shale is 

 indurated by a copious mixture of sand, so as to approach to sand- 

 stone ; in some, it is hardened by oxide of iron; and in not a few, 

 it derives its hardness from an increase of calcareous matter, which 



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