57 
considered it impossible to establish any co-relation. And it is. 
not surprising that he found it even more impracticable to 
establish any co-relation between Somerset and Belgium, except 
in the following general particulars :— 
South West- 
Wales Somerset Hainaut Liege phalia 
Feet Feet Feet Feet Feet 
Thickness of Strata ... 11,000 ... 8,400 ... 9,400 ... 7,600 ... 7,218 
Number of Seams... WOb tant ry ee SUL se 8 .. 117 
Thickness of Coal... 120 ... OR pee RO wus pee beh eeouuy, aoe 
Professor Prestwich naturally remarks on the greater number 
of seams and greater thickness of coal in these Continental Coal- 
fields as compared with the English Measures, which he attributes. 
to the fact that the Pennant Rock, which here is of great 
thickness, and contains but little coal, is i ia by productive: 
Measures in Belgium. 
Although, in the absence of the Pennant, the threefold 
arrangement of strata does not hold good on the Continent, it 
would appear from the information supplied by Dr. Hovelacque 
that the coal beds of the North of France have been divided into. 
groups, distinguished partly by their mineralogical character, and 
partly by the fossil plants which they contain. 
The mineralogical distinction between these groups consists in 
the fact that one is non-bituminous, another semi-bituminous, a. 
third bituminous, and a fourth highly bituminous ; but, judging 
from experience in South Wales, no reliance can be placed on 
any method of grouping which depends on the mere chemical 
character of the coal, as the same seams and groups of seams 
often present a very different character on opposite sides of the 
same basin. 
The system of grouping based on the fossils associated with 
the different seams necessarily rests on a surer foundation, and 
M. Zeiller, an eminent Geologist, has thus divided the Coal 
Measures of the North of France into three zones, called, the 
Upper Coal Measures (Houiller Supérieur and Supérieur inférieur), 
