Yol. 62.] ON THE CARBONIFEROUS ROCKS AT RUSH. 323 



furnished an excellent object-lesson in rock-structure, and suggested 

 such constant repetition, that he wished to ask whether the thickness 

 assigned to the Rush Slates might not be exaggerated. The nodnlar 

 limestone strongly suggested the folding of thin bands on their axes 

 of contortion and in-sheared with the slates. He also commented 

 on the apparent passage of limestone in beach-reefs into slates with 

 chert and decomposed limestone-bands in the cliff, as suggestive 

 of the sudden thickening of limestone -bands, and this tended to 

 deprive lithological character of some of its value as an indicator 

 of horizon in the Carboniferous System. 



Dr. E. A. Bather enquired whether the term Avonian was 

 intended to apply merely to the Carboniferous Limestone Series of 

 the South- West of England, or whether it was intended to denote 

 a division of Carboniferous time. If the latter, then how was the 

 term to be justified when there already existed a 'Bernician Epoch,' 

 including the Visean and Tournaisian Ages ? 1 



Dr. Vaughan thanked the President for his kind remarks. In 

 reply to Dr. Bather, he pointed out that the term Avonian had 

 already been defined in his earlier paper dealing with the Bristol 

 sequence. 2 As a stratigraphical term, Avonian denotes the whole 

 series of deposits, of which the zones from Cleistopora to Dibuno- 

 phyllum are the constituent parts. As a necessary consequence of 

 its faunal basis the term is also an index of relative time, and 

 Avonian time at any place denotes the time during which all the 

 zones from Cleistopora to Dibunophyllum were being deposited at 

 that place. The introduction of such a term satisfies a long-felt 

 want, seeing that ' Carboniferous Limestone ' lacks all definiteness. 

 In the South- Western Province ' Carboniferous Limestone ' only 

 denotes a portion (and that a variable portion) of the sequence. 

 Outside the South-Western Province the use of the term is hope- 

 lessly misleading, since it suggests a correlation which is often 

 entirely false (for example, the ' base of the Carboniferous Limestone' 

 may be a deposit of Dibunophyllum-age). 



Dr. Matlet stated that he had invited the attention of geologists 

 to the peculiar stratigraphy of the limestone-bands in the Rush 

 Slates, as he thought that they might throw some light on the 

 structure of ' limestone-knolls ' referred to by the President. He 

 hoped, before long, to complete the examination of the coast-section 

 nearer Skerries, and then he might be able to ascertain definitely 

 the horizon of the second conglomerate near that locality, which he 

 thought at present belonged to a horizon higher than that of the 

 Rush Conglomerate. 



In reply to Mr. Ussher, the speaker stated that the thickness of 

 the beds as shown in the vertical section was thought to be approxi- 

 mately correct ; but, owing to the obscurity of the dip in places, 

 some modification of the estimate given in the paper might be 

 necessary. 



1 See Eenevier's ' Chronographe Geologique,' 2nd ed. 1897. 



2 Quart, Journ. Geol. Soc. vol. lxi (1905) p. 2(54. 



