Graptolites in the base of the Lower Silurian. 347 



slate full of graptolites, at the same time, that evidence is accumu- 

 lating in Canada, which tends to prove the existence of graptolitie 

 slates associated with the Calciferous Sandrock. 



On comparing the Lower Silurian of England with that 

 of America, it is found difficult to point out in the vast 

 column of the British strata, the horizon representing the 

 base of the limestones of the Champlain group. All that can be 

 said on this question is, that there are about twenty species of fos- 

 sils common to the Lower Silurian of the two countries, and that 

 they all occur in England in the upper half of the Llandeilo and in 

 the Bala group. None of them are found in the lower half. The 

 place of the Calciferous Sandrock would appear thus to be about 

 halfway down the immense depth of the Llandeilo formation. But 

 graptolites are found far below this level in England. The Skid- 

 daw slates for instance are described by Prof. Sedgewick, as, "A 

 group of vast thickness, and probably admitting of several subdi- 

 visions. In some of its upper beds a few graptolites and fucoids 

 have been found. Gi-enerally it is without a trace of fossils. It 

 is the supposed equivalent of the Longmynd Slate, {la) of the 

 Cambrian series.* These slates appear thus to be of the age of 

 the Lower Llandeilo. Yet they hold the following species of 

 graptolites. 



G. sagitarius. (Hisimger.) 

 G. tenuis, (Portlock.) 

 G. latus. (Mr. Coy.) 



Associated with these are compound graptolites allied to species 

 found at Quebec.f According to Prof Hall, G. sagitarius and 

 G. tenuis are found at Norman's kill along with G. serratulus in 

 slates which he considers to belong to the Hudson Biver group. 

 I fancy that no British Geologist would think of placing the 

 Skiddaw slates at the top of the Lower Silurian. 



Again in the lowest slates of the Llandeilo, near the Stiper 

 Stones in Shropshire G. geminus, G. pristis, and G. Murchisonii 

 occur associated with Theca simplex (Salter), a species scarcely 

 distinguishable from a Theca of which I have specimens from the 

 Dikellocephalus sandstone of Wisconsin. These slates belong to 

 tlje very base of the Lower Silurian, and repose upon the Lingula 



* British Palaeozoic Fossils. Intr. p. xxi. 



t See Salter's note " On new fossils from the Skiddaw Slates." Geo- 

 logist, Vol. 4., p. U. 



