306 KEt'ORTW ON tMe S'rATE OF SCIENCt. 



The general dip being N.E., trials for lead along the lower part of the 

 hill and elsewhere point to faults which have not been mapped. We 

 know that at the Grange Quarrj, HoUoway, and Treloggah the chferts 

 rest on the Ci/athaxotiia beds, and the question is to what conditions the 

 absence of chert immediately above the Ci/athaxonia beds at Prestatynj 

 McLaren's Quarry, and at Teilia are due. 



Two solutions at least are pl'obable. 



One, that the cherts at Holloway and Treloggan and Pentre are tile 

 fecjuivalents of the lower portion of the Teilia limestoiles, the CaCOj 

 having been replaced by SiO.2. 



In favour of this view the following facts mdy be aclvaiicbd : — 



1. The position of thfe cherts and FosidonoDiya beds Avith regard to tht; 

 CJyathaxonia beds. 



2. The thin-bedded and tinely stratilifed character of the Pdnidonomi/a 

 limestones and the cherts. 



.3, At the Grange Quarry shales arid thin black limestOntes ocbut- ill 

 the cherts with a Pendleside fauna. 



4. The Pendleside Series at Teilia and Pi-estatyn is more calcareous and 

 contains much less carbonaceous matter than the series known as the 

 Holywell Shales. 



On the other hand, the fossils that have been obtained in the clierts, 

 few though they are, have not a Pendleside facies, especially the fauna 

 obtained from the cherts of Pentre Halkin, which succeed the CyatJiaxonia 

 beds of Waenbrodlas, about three miles south of Holywell. 



Waenbrodlas Quarry shows the Upper Dibunopliyllum beds and 

 Cyathaxonia beds succeeded by from 70 to 100 feet of cherts. In the 

 upper part of these cherts several fish teeth occur which have an Upper 

 Dibunopliyllum facies rather than a Pendleside. 



Dr. Smith Woodward recognised a Cochlodont tooth and a specimen 

 of the tooth of Delioptycldus. 



Several brachiopods are present, amongst which nveSjnri/h'ina biplicata 

 and Dialasina ficus, Productus scabriculus, P. aff. laxispina, all of which 

 Dr. Vaughan has determined. The cherts arc often crinoidal, more so 

 than any bed of the Pendleside Series. 



The C yathaxooiia limestones and the IFpper Dibunophyllum beds at 

 Waenbrodlas and elsewhere on Halkin Mountain are very cherty and 

 contain a large percentage of SiO^. It is therefore quite certain that the 

 replacement of silica at this horizon Avas very local, because these beds at 

 Holywell, The Grange, Treloggan, Prestatyn, and Gwaenysgor are prac- 

 tically free from it. 



It may be, therefore, that at the Grange Quarry and Treloggan the 

 lowest beds of the Pendleside Series have the CaCO;, replaced by SiO.^, 

 while nearer Prestatyn and Teilia only beds somewhat higher in the series 

 have undergone this chemical change. In connection with this view the 

 great variation in the thickness of chert must be remembered. 



The other explanation is that the chert beds have been cut out at 

 Teilia and Lady McLaren's Quarry by unconformity or by faulting. The 

 ground is difficult ; faults are numerous and difficult to trace owing to the 

 amount of drift and paucity of sections. 



In favour of this view is the fact that Posidonomya Becheri and the 

 other fossils which occur with it at Teilia and Prestatyn have never been 

 found in the cherts. 



