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is to this that the for- 

 mation owes its trail s- 

 gressive relations to the 

 underlying- beds, and also 

 its peculiar lithological 

 features. The sea was evi- 

 dently subject to the influence 

 of strong currents which rolled 

 up the ashy mud of the sea- 

 floor into halls that were 

 carried along, and dropped 

 elsewhere among sandy and 

 ashy deposits. Somewhat 

 similar structures may he 

 found in certain beds of the 

 Carboniferous Limestone, in 

 which deposition has been 

 simultaneous with contem- 

 poraneous erosion elsewhere. 

 Conglomerates which consist 

 of pebbles of marl in a 

 matrix of sandstone are known 

 both in the Old Red Sand- 

 stone and in the Trias. 



It is further evident that 

 Gelli-llwyd is situated on what 

 was the western edge of the 

 volcanic pile. This is indi- 

 cated by the fact that it is 

 precisely at the point where 

 the rhyolitic rocks increase 

 in thickness rather suddenly, 

 that the transgression of the 

 Bryn Brith Beds takes place. 

 Where was the eastern limit 

 of the volcanic pile cannot be 

 ascertained until the exami- 

 nation at present in progress 

 by one of us (A. K. W.), of 

 the country east and north- 

 east of Dolgelley has been 

 completed in more detail. 



(iii) The C r g e n e n 

 Slates.— The Crogenen Slates 

 form the best-defined of all 

 the slate-bands among the 

 lower part of the Ordovician 

 sequence. They succeed the 

 coarser strata of the Bryn 

 rather abruptly, especially so in the western part of 



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