126 J. E. Marr—The Sldddaw Slates. 



ordinary Arenig Grraptolites of the area, it could hardly have 

 escaped detection, for it must have been far from rare considering 

 the large numbers observable on the two slabs which I have seen, 

 and slabs with Arenig Graptolites have been collected by hundreds. 

 It is possible that some of the other compound branching Grap- 

 tolites of the Skiddaw Slates may also be of Tremadoc age. Some 

 forms have been doubtfully referred to Clonograptus, which occurs 

 in Tremadoc rocks, but none which I have seen are specifically 

 identical with Tremadoc species, and it is doubtful whether any of 

 the forms are truly referable to the genus. 



2. Akenig-Llanvirn Beds. 



A. Dichograptiis Beds. — The evidence upon which these beds are 

 separated from the newer Tetragraptiis Beds is unfortunately of a 

 negative character, though the cumulative evidence is fairly con- 

 vincing. In the first place, the general rule appears to be for the 

 more numerously-branched forms of DichograptidcB to occur in 

 earlier beds than those with fewer stipes. When the evolution of 

 the Dichograptidcs is worked out in full, I have no doubt that the 

 reason for this will become evident. Again, Dichograptus, Logano- 

 graptus and Temnograptus are never found in association with the 

 Tetragrapti of the Skiddaw Slates, i.e. upon the same slab. Lastly, 

 there are areas where Tetragraptus is found in which Dichograptus 

 does not occur, e.g. in the Arenig rocks of South Wales and the 

 Bennane shales of the Ballantrae District' : as Dichograptus and 

 Loganograpttis are widely distributed forms, this would hardly be 

 the case if they existed at the same time, and the probability is that 

 the Dichograptus Beds are not represented, or have not been dis- 

 covered, or are represented by non-Graptolitic Beds in South Wales 

 and South Scotland. 



B. Tetragraptus Beds. — These beds contain by far the larger pro- 

 portion of Graptolites which have been obtained from the Skiddaw 

 Slates. In the list of Graptolites above given, all the species of 

 Tetragraptus, Pliyllograptus, Didymograptus, Azygograptus, etc., 

 which have other localities assigned to them than those on which 

 Ellergill Beds only occur, are found in the Tetragraptus Beds. Of 

 the Graptolites appertaining to this horizon — 



Tetragraptus bryonoides occurs on the same slab as T. Bigshyi. 



^j ,, occurs on the same slab as JDidymograptus gibberulus. 



,, Bigsbyi occurs on the same slab as Didymograptus indentus var. nanus. 



,, bryonoides occurs on the same slab as Phyllograptus typus. 



,, quadribrachiatus occurs on the same slab as Didymograptus nitidus. 



,j ,, occurs on the same slab as T. crucifer. 



Didymograptus nitidus occurs on the same slab as Dicellograptus moffatensis. 



The Graptolites of this division are usually preserved in black 

 slates, but others are found in slates of a lighter colour, or even in 

 light-coloured gritty shales. 



The Tetragraptus Beds are probably divisible into sub-zones. 

 For instance, Azygograptus Lapworthi occurs in great abundance in 



1 Cf. Lapworth and Hopkinson, Quart. Journ. Geol. Soc. vol. xxxi. p. 631 ; and 

 Lapworth, Geol. Mag. Dec. III. Vol. VI. p. 22. 



