1864. | Hut on the Coal Resources of Great Britain. 25 
or “fields:” and on the threshold of our inquiry it may be well to 
give a short sketch of each of these systems. 
Fic. 1.—Section of the Forest of Dean Coal-Basin. 
1. Coal-Measures. 2, Millstone Grit, 3. Mountain Limestone. 
Coal-basin.— The section of a coal-basin is represented in the 
above woodeut. The term is used when the beds dip from every part 
of the circumference towards the centre. When the basin is elongated 
in one direction to a considerable degree, it is called a “trough ;” 
but as it is rare for any coal-bearing tract to be even approximately 
symmetrical, the term “basin” serves to denote all such tracts, 
whether the outline be circular or oval. To this form belongs the 
largest coal-tract in Britain—the South Wales Coal-field (No. 23 in 
_ Map), as also that of the Forest of Dean (24), and several others. 
Fic. 2.—Section of the Yorkshire Coal-Field. 
Q Ss < = SSSS = = SSS = 
6 oa 4 3 
1. Magnesian Limestone, 3. Coal-Measures. 5. Limestone Shale, 
2, Permian Sandstone. 4. Millstone Grit. 6. Mountain Limestone. 
Coal-field.—In the case of a coal-field, the strata dip (with more 
or less regularity) in one direction. Such an arrangement has many 
modifications; either the strata dip under those of a more recent 
formation, as in the case of the Yorkshire Coal-field (Fig. 2), or they 
are cut off along one side by a fault, as in the Anglesea coal-field. 
This is the more general form which a coal-tract assumes, and is 
often much varied by rolls in the strata, or by dislocations. 
Coal-group.—Where the strata of several coal-fields dip towards 
each other, and under those of a newer formation, such as the New 
Red Sandstone, it may generally be inferred that they are connected 
underneath, and that if the newer formation were penetrated, the coal- 
measures could be reached beneath. When several of these coal- 
fields are thus physically connected, they give rise to what may be called 
“a group of coal-fields,’ or simply a “ coal-group.” Under the same 
title we also place a number of distinct basins or fields, which were ori- 
ginally connected, but have since been dissevered by denudation, as 
those of the central valley of Scotland. In this manner the British 
coal-areas naturally arrange themselves into four groups, which, on 
the map, have been marked as the Northern, Western, Eastern, and 
Southern coal-groups. These great divisions refer more immediately 
to the present arrangement of the tracts than to that which they 
assumed at the time of their formation. Nevertheless there is reason 
to believe that out of the four only two were originally continuous 
with one another, namely, the eastern and western groups. From this 
