128 J. E. Marr—The SMddaw Slates. 



District. Unfortunately their fauna is almost unknown, Didymo- 

 graptus MurcMsoni and Diplograptus dentntus being the only fossils 

 hitherto recorded from them. They probably represent the upper- 

 most part of the Arenig or the lowermost part of the Llandeilo 

 formation. It is to be hoped that local observers will pay special 

 attention to the fauna of these beds, as the exact determination of 

 their age is a matter of considerable importance. 



From a study of the Skiddaw Graptolites, then, we obtain the 

 following provisional classification of the Graptolite-bearing portion 

 of the Skiddaw Slates : — 



fd. Milburn Beds = Uppermost Arenig or Lower Llandeilo. 

 c. Ellerffill Beds. 



lis nanus. 



2j b. Teiragraptus Beds|]^PPf; ^^^^ Diclymograpt. 



^a. Bichograptus Beds. 

 1. Bryograptus Beds = Tremadoc Slates. 



An examination of sections drawn through the Skiddaw Slates, 

 as, for instance, those constructed by the Officers of the Geological 

 Survey, will show that these slates are greatly contorted. The con- 

 tortion is often more violent than is represented on those sections. 

 Study of natural sections of the Ellergill Beds, for example, shows 

 that these deposits are constantly affected by overfolds having the 

 middle limbs faulted out, and the same thing can be frequently 

 detected in other parts of the series. The distribution of the fossils 

 shows that little reliance can be placed on dips as an indication of 

 the order of succession. For instance, the fossils obtained from the 

 screes of Barf show that the Br%jog%-aptu8 Beds, the Dichograptus 

 Beds, and the two divisions of the Tetragraptus Beds, must be 

 developed in situ in that hill, or, in other words, that part of the 

 Tremadoc and the greater portion of the Arenig Series is included 

 in one or two hundred feet of rock. The same beds, with the 

 exception of the Bryograptus Beds, are found on Outerside, where 

 the beds ai'e not on the same line of strike with those of Barf. 

 Kepresentatives of a considerable portion of the Arenig Series must 

 also occur at Carlside Edge, and at several other localities. The 

 thickness of the Ellergill Beds cannot be very great, and the 

 probability is that the whole of the Graptolite-bearing portion of the 

 Skiddaw Slates is not many hundreds of feet in thickness. Mr. Ward 

 calculated the total thickness of the Skiddaw Slates at from 10,000 

 to 12,000 feet. This is probably an over-estimate, but the whole 

 series must be far thicker than that portion containing Graptolites. 

 Great masses of gritty rock occur which contain no Graptolites, and 

 to the north of Skiddaw there is a considerable spread of red and 

 green shales, which ^.re very unlike the Graptolite-bearing Skiddaw 

 Slates, but resemble beds referred to the Skiddaw Slates at Kake 

 Beck, near Melmerby, in Teesdale, and at Ingleton. As these 

 cannot be newer than the Graptolite Beds of the Skiddaw Slates, 

 which pass up into the volcanic rocks of the Borrowdale group, they 

 must be older. The Graptolitic Beds are, in fact, comparatively 

 thin masses of deposit nipped in amongst older rocks in broken 



