352 SECTIONAL TRANSACTIONS.— C. 



groups of tuff and agglomerate with spilitic rocks and mudstones. 

 Amplexograptus perexcavatus and Dicellograptus sp. occur. 



The Upper Rhyolitic Group (i ,500 ft.). Caradoc (?). — This, like the Upper 

 Acid Group of Cader Idris, consists mainly of massive and streaky rhyolitic 

 tuffs. It is followed by the Ceiswyn slates and mudstones. 



Sills of dolerite affect each of the rock series named and occur as high as 

 the summit of the Upper Rhyolitic Series. Intrusive rocks of acid com- 

 position are represented. 



Discussion on Coal Measure correlation (10.30). 



Prof. A. E. Trueman. — The correlation of the Coal Measures. 



Problems of correlation in the Coal Measures involve both the recognition 

 of zones and the identification of individual horizons. In comparing 

 sequences in different coalfields, in recognising strata met with in new 

 explorations (especially of concealed coalfields) and in the interpretation of 

 structurally complex areas, the identification of floral or faunal zones is of 

 outstanding importance ; the recognition of datum planes and correlation 

 on a more detailed basis is more often necessary in actual mining operations. 



Within the last ten years work on the faunas and floras of the Coal 

 Measures and on the coal seams has provided a basis for the solution of 

 most of these problems. The charge of neglecting the stratigraphy of that 

 formation which is of most economic importance can no longer be justified. 



In many instances minute palaeontological studies have confirmed the 

 correlations which had been based on thicknesses and on comparisons of 

 sections, but in other cases erroneous conclusions had been drawn from such 

 data. The broader divisions made by palaeobotanists probably hindered the 

 development of real floral zones. 



The peculiar conditions under which the Coal Measures were laid down 

 and the surprising uniformity of many horizons (coal seams, marine bands, 

 etc.) over wide areas facilitate correlations even when the fossils would at 

 first sight be thought unsuitable for detailed work. 



Dr. W. B. Wright. 



The position as regards the correlation of the Coal Measures of Great 

 Britain has improved greatly during the last ten years and there is now a real 

 hope that fairly precise identification of horizons may ultimately be attained. 



An important advance at the beginning of this period was Prof. Trueman's 

 revision of the non-marine lamellibranchs. The zones established by him 

 are, however, very broad and identifiable with difficulty. Their limits are 

 moreover indefinite and although a general correspondence in the succession 

 can be traced from coalfield to coalfield, little precision is obtainable by their 

 application. One zone in particular is wanting in the north. 



Within the limits of the Lancashire field a more accurate result has been 

 obtained by a system of subzones. These lead to exact identification of 

 seams only in two or three cases, not always accessible. The range of error 

 in local correlation by subzones varies from precision up to several hundred 

 feet, even where material is available. As regards correlation with other 

 fields equivalence is suggested by the subzonal forms on a few horizons but 

 not accurately. 



For exact correlation both locally and between fields something else is 

 required, and recent researches into the spore-content of coals afford 



