C.—GEOLOGY 67 
in an estuarine or lacustrine series would be equally valuable. It happens 
that the latter types of rock may contain coal seams, and, consequently, 
marine bands have assumed a very great importance in correlating beds 
in different areas. But a widespread ash bed would have been quite as 
useful for mapping.** For wider zoning plants have proved useful. 
But Kidston’s classification is not expected to stand as the last word, 
and already research has shown it defective. Dr. Emily Dix *4 believes 
that the base of the Staffordian Series should be lower than Kidston 
suggested ; and she finds that, with the line drawn at the lower level, 
it will more or less correspond with one of the molluscan zones—that of 
Anthrocomya phillipsi. Any classification that will bring diverse groups 
into accord is of the greatest value. Dr. Dix has made another suggestion 
that might well be examined carefully. I have stated above that it is the 
accidental character of marine horizons in a non-marine series and the 
accidental occurrence of an ash bed that give them their value in strati- 
graphy. Dr. Dix states: ‘. .. that more attention would have to be 
paid to the vertical ranges of various species of plants, and in particular 
to the occurrence of rare species, which often give a clue to a particular 
horizon, before an ideal classification could be made.’ This suggestion 
_ should be followed up, and the result might remove the odium with 
which some mining engineers regard fossil plants. 
FossiL PLANTS AS QUANTITATIVE AND QUALITATIVE INDEXES. 
Another phase of research in plant palzontology is the quantitative 
type which has been explored along two lines: (a) for correlation of strata 
in a comparatively small area with possible extension over a wider field, 
and (bd) for correlation of one and the same coal seam over large or small 
areas. In 1929 * the late Mr. David Davies published the results of many 
years’ work in an area of some 30 square miles in East Glamorganshire. 
The most distant points were 5 miles apart. He recorded the colossal 
number of nearly 400,000 fossil plants, and, as a consequence, obtained 
a very accurate idea of the quantitative balance of plants at different 
horizons (actually 29) in the Coal Measures of that district. The strata 
searched were normally the shales immediately on top of the coal seams, 
but sometimes from 2 to 14 ft. above these seams. On the whole, there- 
fore, the flora, or microflora, is that associated with the individual seams. 
‘His results show that the floras ranged from wet to dry types. In the 
former lycopods (Lepidodendron, Sigillaria, etc.) predominate, and in 
the latter ferns and pteridosperms. Calamites and Cordaites were dis- 
tributed fairly evenly throughout both types. Molluscan remains 
(Lamellibranchs) occurred abundantly in association with the lycopods, 
ie. the wet type. On the whole the dry floras were the more common, 
and so he concludes that coal, in these seams, is not so much a swampy as 
a drier boggy accumulation. I am convinced that a continuation of his 
88 Dr. Ellis has actually used an ash bed, the Frondderw Ash, in mapping the 
rocks round Bala. Q.J.G.S., vol. Ixxviii, 1922. 
% EF. Dix, ‘ Coal Measures of North Staffordshire,’ Q.].G.S., vol. Ixxxvii, 1931. 
% Phil. Tvans. Roy. Soc., vol. ccxvii, 1929. 
