BRITISH AVONIAN CONODONT FAUNAS 31 
The general geology of the Scottish collecting areas is described in the following 
publications : Carruthers et al. (1927), Clough et al. (1925), Craig (1965), Currie 
(1954), Dinham & Holdane (1932), Goodlet (1957), George (1958), Mitchell & 
Mykura (1962), Richey e¢ al. (1930), Robertson et al. (1949) and Tulloch & Walton 
(1958). 
IV. METHODS OF STUDY 
Samples were collected from the successions at intervals of 10 ft. and in many 
parts of the sections these were supplemented by collections made at 5 ft. or 2 ft. 
intervals. Most of the original samples from the reconnaissance survey weighed 
about 25 lbs. but the samples from the smaller intervals weighed only 2—4 lbs. 
These samples were weighed and then digested in 8% commercial acetic acid, 
contained in plastic buckets. Shales were disintegrated with concentrated hydro- 
gen peroxide, and formic acid was used for some of the limestones. After the rock 
had dissolved, the sludge was sieved under water on 25, 50, 100 and 200 mesh 
screens, the fractions being dried and then separated in bromoform. 
The heavy residues were searched with a binocular microscope and the conodonts 
picked and mounted (see Collinson 1963 for further details). 
Photography was carried out by Mr. S. Osborn, using a Leitz Aristophot apparatus 
and Adox KB 14 film, developed in Acutol or I.D. 48 developer. The specimens 
were coated with ammonium chloride. Prints were cut out and mounted but no 
retouching was undertaken. 
V. CONODONT FAUNAS 
(a) General Review 
The present study is based upon 600 rock samples, and has involved the solution of 
some 3 tons of rock, which yielded over 25,000 identifiable conodonts. We regard 
this total collection as satisfactory, although the average abundance of conodonts 
per kilogram of rock is considerably lower than the yields from some other areas. 
The distribution of conodonts within the various sections studied was far from 
uniform, and abundance ratios for each sample are included with each of the litho- 
logical sections (Figs 59-92). The succession in the Avon Gorge illustrates the 
problems involved in the less fossiliferous parts of the section. The average number 
of conodonts per kilogram was 8 for the K Zone, with a range from o—47 per kilogram. 
In general the shales provided poor yields or were barren, while intervening lime- 
stones were relatively fossiliferous. Shale samples 7 and 8, for example, were barren, 
but they are separated by two thin limestones, yielding 20 and 16 conodonts per 
kilogram. 
The Z Zone consists predominantly of massive, blue-grey calcarenite, and has an 
average yield of 19 conodonts per kilogram. There is again a fair range of variation 
_ (0-66 per kg.), the lowest yields being in limestones with a high haematite content. 
The upper beds of the Zone had high yields (45-66 per kg.). 
In contrast, the Lower C,, the Laminosa Dolomite, yielded an average of only 3 
conodonts per kilogram, probably reflecting either the destructive results of second- 
