C— GEOL0C4Y. 69 



in the same sense as heretofore, not binding themselves by adherence to 



the Continental usage. 



Note. — The chief differences between the British and Belgian use of the 



terms Tournaisian and Visean, and between Avonian and Dinantian, 



are : 



1. In Belgium the K beds are excluded from the Dinantian and 

 grouped with the Famennian. 



2. The line between Tournaisian and Visean is drawn in Belgium 

 at about the middle instead of at the base of C.^. 



3. The top of the Dinantian is drawn at the base of the Yoredale 

 equivalent, that of the Avonian at th»^ top. 



Variability and Persistence. 



In my remarks on this subject the Northern Province is not taken 

 into account. 



The several horizons of the Avonian difier much as regards variability 

 and persistence. 



The Cleistopora beds are both the lowest and most constant horizon. 

 Throughout the Bristol area, South Wales and the Forest of Dean, where 

 alone they are present, they show remarkable constancy not only in fauna! 

 and lithological character but in thickness. Metasomatic changes such 

 as dolomitisation and silicification have affected them very little, and 

 while in parts of the South Wales coalfield all the other horizons are 

 concealed by overstep of the Millstone Grit, the Cleistopora beds are never 

 reached. 



The Zcvphrentis beds, wliich are also comparatively little developed in 

 the region under consideration outside the South-west Province, are 

 relatively constant in lithological character. They are predominantly 

 crinoidal limestones of the ' petit granit ' type, but may sometimes be 

 highly dolomitised or silicified. They are concealed by overstep in parts 

 of the South Wales coalfield. 



The Syringothyris beds, the oldest strata commonly seen in the North- 

 west Province, show much variability. In the south they contain much 

 dolomite, and the whole or part may be cut out by non-sequence or con- 

 cealed by overstep as in the South Wales coalfield. 



The Seminula beds, the oldest strata occurring in the Midlands or North 

 Wales, are also a variable series. In the Bristol district they are on the 

 whole more frequently exposed than any other horizon. 



The limestones of the lower Dibimophyllum zone (Dj) are the most 

 constant limestones in faunal and lithological characters of the whole 

 succession. Throughout the whole of the Bristol district, North and 

 South Wales, the Midlands, Yorkshire and the North-west Province, 

 wherever limestones of this level occur, they are of the same general type, 

 save in certain districts where the Knoll facies is found. The Avonian 

 rocks of later date than the D, beds are much more variable. 



Relations ot the Carboniferous to the underlying Rocks. 

 Wherever the Carboniferous and Devonian (O.R.S.) rocks are seen 

 in relation to one another the succession is conformable save in Carmarthen- 

 shire, at Kidwelly and Llandebie, where, as Mr. T. N. George informs me, 



