Vol. 55.] THE CBAVEN DISTKICT OF YOEKSHIRE. 355 



character of the limestone, which is usually white, light grey, or 

 pink, the last-named colour being probably due to subsequent 

 staining ; and (v) frequent dolomitization and silicification. I may 

 remark that in all the cases that I have examined the knoll- 

 structure is developed only in comparatively pure limestone, while 

 associated argillaceous limestones, even when found under conditions 

 similar to those which furnish knoU-structure, do not seem adapted 

 for its development in a state approaching perfection. 



Commencing with an account of knoll-structures developed in 

 our own country, I have notes of its occurrence in the Coniston 

 Limestone Series of Keisley in Westmoreland, Millom in Cumber- 

 land, and in the neighbourhood of Dalton-in-Furness. I have alluded 

 to the existence of a white horny limestone lying between the 

 ordinary Applethwaite Limestone and the Staurocephalus-limestone 

 of the Coniston Limestone Series,^ and suggested that the Keisley 

 Limestone, which probably occupied this position (a suggestion 

 since confirmed by Mr. F. E. C. Eeed) was identical with this 

 limestone. This is probably also the case with the white crystalline 

 limestones of Dalton-in-Furness and Millom, as the ordinary impure 

 ashy Coniston Limestone is seen in its normal condition close to 

 these, and it would require the abstraction of a considerable amount 

 of insoluble material to convert this limestone into the comparatively 

 pure crystalline limestones of those localities. The Keisley Lime- 

 stone shows evidence of much disturbance and uneven bedding. 

 Prof. Nicholson and I wrote of it elsewhere : — * The limestone itself 

 shows signs of much disturbance : it contains twisted wisps of shale 

 in places, and the occurrence of beds containing numerous Illceni with 

 their convex surfaces uniformly pointed downwards indicates 

 inversion.' ^ It further resembles the knolls of Craven in the 

 crystalline character of the rock, the sporadic distribution of fossils, 

 and occasional dolomitization. It is surrounded by faults, and 

 occurring as it does between hard rhyolitic rocks on one side 

 and the hard bands of the upper part of the Stockdale Shale 

 Series on the other, it is in a position eminently adapted for 

 having the knoll-structure impressed upon it. Mr. F. E. C. Eeed,^ 

 while partly accepting my views as to the origin of the Keisley 

 'knoll,' does not appear to consider them sufficient, and puts 

 forward a theory of discontinuous distribution to account for the 

 fauna, with hints at migrations. If his view be accepted, after 

 the migration, and during the occupation of the isolated regions at 

 Keisley, Kjldare, and elsewhere, either the intervening seas were 

 untenanted by organisms, or else they were tenanted by an earlier 

 or a later fauna than that of the Keisley Limestone, or a mixture 

 of the two. Under the circumstances, it is difficult to see why the 

 fauna of Kildare should be so like that of other places. Discon- 

 tinuous distribution is a convenient escape from the difficulty, but 



^ G-eol. Mag. 1892, p. 100. 



2 Quart. Journ. Geol. Soc. vol. xlvii (1891) p. 507. 



3 Ibid. vol. liii (1897) pp. 100 et seqq. 



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