1862.] HARKNESS SKIDDAW SLATE SERIES. 133 



beds of the series are exposed. This conclusion shows that Grapto- 

 lites occur very low down in the series, — an inference which is justi- 

 fied by the sections of these rocks as seen in other localities. 



§ 7. Conclusion. 



The lithology of the Skiddaw slates varies considerably in the 

 several areas where they occur, and, with reference to geological 

 position, there is in some cases a difference in their mineral nature. 

 This, however, is not of so uniform a character as to admit of this 

 series being divided into subgroups marked by mineral aspects. The 

 fossil contents are even less applicable for subdivisions than mineral 

 nature, since, as already stated, the same organisms are disseminated 

 through the whole of this portion of the Lower Silurians as they 

 occur in Cumberland and Westmoreland. Under these circumstances, 

 I am induced to look to other evidence for relative position among 

 the strata forming the Skiddaw slates. 



The physical geology of the various areas under consideration 

 shows that in all of them, except that which lies west from Derwent- 

 water and Bassenthwaite Lake, axes are by no means well exhi- 

 bited. Even in that large area south of Caldbeck-fells there is no 

 well-defined axis, but a continuous S.S.E. dip from the northern 

 limits of the Skiddaw slates almost to their southern margin. The 

 area west of Derwentwater and Bassenthwaite affords two well- 

 marked axes ; and the nature of the rocks between them shows that 

 the Skiddaw slate series is made up of fine indurated shaly beds 

 with well- developed slaty cleavage, having intercalated through them 

 coarser strata almost devoid of cleavage, possessing a flaggy nature, 

 and affording fossils. 



From Newlands, where the upper members of the Skiddaw slates 

 are seen with the superposed greenish-grey rocks, to Coldale, where 

 the most southern of the axes just alluded to occurs, the distance is 

 less than three miles measured along the dip, which averages 30°. 

 This portion of the section (see fig. 2, p. 116) would give a thickness 

 of Skiddaw slates, from the greenish-grey rocks above to the lowest 

 beds exposed at Coldale, of about 7000 feet ; and this is probably 

 their average thickness where best seen in the north of England. 



The thickness of the succeeding greenish-grey rocks is not so 

 easily made out. There are, however, circumstances which render 

 it probable that these latter rocks do not exceed in thickness those 

 of the Skiddaw slate series ; so that between the base of these latter 

 and the Coniston limestone, the equivalent of the Bala limestone, 

 there is a varied mass of rocks of 14,000 feet in thickness, the lower 

 half consisting of rocks of a sedimentary nature, while the upper 

 half is composed of rocks which had an igneous origin, and which 

 consist principally of porphyries and ashes. This thickness of rocks, 

 which in Cumberland and Westmoreland lies below the Coniston 

 limestone, corresponds with the thickness of the Llandeilo flags*. 



The Skiddaw slates yield some forms of Graptolites which occur in 

 the southern Highlands of Scotland, indicating some palaeontological 

 * < Siluria,' 2nd edit, p. 194. 



