^^^' 5 5 J GEOLOGY OF NORTHERN ANGLESEY. 643 



recentl}^ he has recorded undoubted Arenig fossils (Didymograptus 

 ^atulus, D. nitidus, D. extensus ?, Climacogniptus ?) from the Caer- 

 narvonshire side of the Menai Bridge/ 



The sole evidence for assigning the basal Protozoic rocks of Central 

 Anglesey to a Tremadoc horizon is the occurrence of the fragmentary 

 and disputed trilobites mentioned on p. 642, and the presence of 

 Orthis Carausii. I have shown that this brachiopod is found a few 

 miles awa}' in Llandeilo Beds. There is a marked absence of typical 

 Arenig graptolites, such as Tetragraptus and Phgllograptus, and the 

 lowest zonal graptolite, Didymograptus MurcJdsoni^ indicates a Lower 

 Llandeilo horizon. The evidence of Tremadoc age should therefore, 

 as Dr. Callaway has already argued, be rejected until further proof 

 be afforded. It is much more likely that the fossil-bearing beach- 

 rocks of Central Anglesey are equivalents of the base of the 

 Llandeilo, or the top of the Arenig, and there is in this area a 

 succession through the Llandeilo and Bala into the Llandovery 

 horizons noted at Treiorwerth and Parys Mountain. 



(iv) Conclusions from the Palseontological Evidence. 



We see, then, that the southern Ordovician contains fossils from 

 early Llandeilo to the Llandovery. The northern Ordovician com- 

 mences rather higher in the Llandeilo, and ranges perhaps into the 

 Bala. The palseontological evidence is therefore distinctly opposed to 

 the theory that the Green Series is of Bala age, interbedded between 

 older strata on the south and newer strata on the north. That theory 

 was first suggested by Prof. Hughes, at a time when fossils from 

 the area were almost unknown, because he thought that it afforded 

 .a simple and natural explanation of the geological structure. But 

 this simplicity of structure is shown by the fossils to be untenable. 

 We have to imagine an almost complete inversion of the strata, and 

 even then we have to account for the mysterious absence of the 

 Green Series between the Llandovery and the basement-rocks of the 

 central area. On palgeontological grounds alone it seems necessary 

 to abandon the theory ; if not, we land ourselves in strati graphical 

 complexities far more improbable than those which the assump- 

 tion of a Bala age for the Green Beds was intended to remove. 



The fossils bring out another point which is worthy of discussion, 

 namely, that the conglomerate in Northern Anglesey seems to be on a 

 higher horizon than the Ordovician base in Central Anglesey ; in 

 the latter area it is below the zone of Didymograptus Murchisoni, 

 in the north it appears to be well above that zone. This may be 

 explained on the reasonable assumption of an overlap to the north, 

 such as would result from a continuance of the conditions which 

 caused the overlap of the successive horizons of the Cambrian when 

 followed from Merionethshire northward. Anglesey seems to have 

 remained dry land throughout the Cambrian period, and if its more 

 elevated portion lay in the north of the present island when it was 

 slowly submerged in Ordovician times, the old beach creeping up 

 the slopes of the sinking land would there be of later date. 



1 Geol. Mag. 1898, p. 561. 



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