C.—GEOLOGY 57 
it fractures when in fragments of sufficient size—affords a means by 
which the proportion of this material in the aggregate can at once be 
roughly assessed. The more vitrinite, the brighter the coal, is a rule 
which is seldom broken. Ever since coal was used the different qualities 
of ‘bright’ and ‘dull,’ or ‘ softs’ and ‘ hards’ have been recognised ; 
and it is the large or small proportion of vitrinite which is mainly in 
question. 
Complete gradation from the brightest coals, consisting entirely of 
large fragments of vitrinised tissue—that is, of vitrain lenticles only— 
through every stage of decreasing particle size, decreasing vitrinite content 
and decreasing lustre, to the most finely granular dull aggregate is to be 
expected and is easily found. But experience readily shows that in most 
seams there is a small proportion of bands which are unmistakably quite 
without lustre, and that these are sharply separated from the ‘ bright’ 
coal, even though the lustre of the bright bands may vary widely. The 
distinction of the two types of aggregate clearly corresponds to some 
difference in the mode of accumulation. The really dull bands are usually 
(though not necessarily) characterised by a very much higher content of 
ash than the bright coal, and this excess always consists of a very fine 
clay which must have been water-borne. It is clearly desirable to dis- 
tinguish this true dull coal from the rest of the seam, and hence the term 
durain applied to it is much to be preferred to the merely relative ‘ dull 
coal,’ which is in fact often applied to some of the vitrinite-rich coal 
which is merely less lustrous than the rest. Durain and clarain are the two 
main types of coal in an ordinary bituminous seam. Both are of variable 
composition, and the one may shade into the other though they are 
commonly somewhat sharply separated. The clarain, however, with its 
dominating vitrinite constituent, tends to comparative uniformity of 
composition, while the durain may be far more varied. A rich content 
of spores, cuticles or resins may make the durain conspicuously rich in 
hydrogen ; while a dominance of residuum, or a high content of frag- 
_ mented fusain, may have the opposite result. It is the variable part of 
the coal. 
The Rank of Coals.—We have now reviewed what microscopic examina- 
tion has disclosed concerning the nature of the various ingredients which 
_ make up the coal deposits, and have glanced slightly at the various types 
of coal aggregate which these ingredients may form. It has always been 
more or less clearly recognised, however, that coals differ not only in the 
type of aggregate of which they may have been compounded, but in the 
degree to which the original composition of this aggregate has been 
altered. This degree of alteration constitutes what we speak of as the 
rank of the coal, we must now consider the respective shares of type and 
rank in determining the final composition of the coal. 
Ever since Rogers drew attention to the gradual change in the quality 
of the coals as one proceeds towards the Allegheny axis or away from it, 
debate has continued on the question as to how far the quality of the 
coals has been determined by the geological conditions to which they have 
been subjected, and how far it is due to the original plant composition, 
conditions of deposition and the manner of organic decay. No one has 
