BRITISH AVONIAN CONODONT FAUNAS 13 
Central Province, lying to the north of this barrier, rocks of Yoredale facies were 
deposited. These include cyclothemic limestones, non-marine sandstones, shale, and 
coal sequences, with a fauna of corals, brachiopods, bivalves, occasional goniatites 
and non-marine plants. Bioclastic limestones and spectacular reefs also developed 
in various places in this region, such as the reef knolls of Derbyshire and the Craven 
Lowlands, and the larger sheet-like apron reefs of Southern Ireland. 
In the Northumbrian trough, lying to the north of the Central Province, as well as 
in the trough of the Midland Valley of Scotland, which was separated from it, a 
distinctive facies of the Cementstone type was developed, consisting of alternating 
thin argillaceous limestones, sandstones and grey-black shales. Fossils are rare and 
include fish, ostracods, inarticulate brachiopods and spirorbid worms. These rocks 
are overlain by sandstones, coal-bearing strata and limestones, the total Northum- 
brian section including some 7,000 ft. of strata. In the Midland Valley of Scotland, 
a broadly similar variety of rock types is found, although there is no detailed 
equivalence in age ; clastic deposits predominate there, and include the Oil Shales 
and the Calciferous Sandstones. There are also thick lavas in places. 
In north-western Ireland great thicknesses of deltaic strata were deposited, which 
pass southwards into limestone-shale and bioclastic limestones (George 1955). 
Rocks of Lower Carboniferous age present formidable problems of correlation and 
these arise largely from the rapid and almost continuous lithological and faunal 
changes which they display. The general problems of correlation have been 
reviewed by Rayner (1953) and by George (1952 and 1958). The first successful 
attempt to provide a palaeontological subdivision of the rocks of the South-West 
Province was that of Vaughan (1905) who proposed the now widely-applied coral- 
brachiopod system for the limestones of south western England. This was based 
upon exhaustive and meticulous collecting of faunas, especially from the Avon Gorge. 
Vaughan established his zonal scheme on the first appearance of particular genera 
and species, although the zones as interpreted today are partly assemblage zones, 
based on the occurrence of a number of species. To a varying extent the demarca- 
tion of Vaughan’s zones was influenced by the marked lithological changes which 
occur in the Avonian strata of south-western England (see p. 17). 
It was early recognized that Vaughan’s zonal scheme was inapplicable to the 
different facies of northern England and in that area the work of Bisat (1924) on 
goniatites provided the basis for much of the present classification. 
Present views on the validity of these various zonal schemes are sharply divided. 
There is general agreement that, in the sense in which they were originally established 
by Vaughan, the coral-brachiopod zones can no longer be applied in detail, but some 
workers, especially Kellaway & Welch (1955), reject the whole zonal scheme which 
they represent. The detailed problems of correlation are discussed on p. 52. 
(b) The Avonian Succession 
The Avon Gorge, Bristol, has long been regarded as the type area for the British 
“ Lower ” Carboniferous. The base of the Lower Limestone Shale was selected by 
Buckland & Conybeare (1824) and by De la Beche (1846) as the base of the Lower 
