C.—GEOLOGY 61 
content ; others contain a larger proportion of durain and (commonly) a 
lower percentage of hydrogen. A careful comparison of the analyses 
shows clearly that the seams which are high in hydrogen retain that 
peculiarity regardless of change in rank, while those which are low remain 
low. In this way the chemical evidence supports that provided by the 
detailed examination of the physical peculiarities of the seams, which 
shows them to have, on the whole, a very constant character throughout 
their range. Still further proof of the same fact is provided by the work 
of Dr. Raistrick and others on the very distinctive spore-content of the 
various seams, which again shows a notable constancy and is quite 
unaffected by the changes in rank. The cumulative evidence leaves no 
room for doubt that in these instances the change of rank is quite 
independent of the original constitution of the seams. 
Hilt’s Law.—The simplest and most familiar evidence of a relation 
between rank and geological conditions is Hilt’s law: that, in any single 
vertical section, the deeper seams are of higher rank than the upper seams. 
In spite of frequent denials, there seems little doubt of the general applic- 
ability of this rule. Apparent failure of the rule in some cases is doubtless 
due to the fact that original differences between seams may affect their 
present composition to a greater extent than small differences of depth. 
But when the differences of depth are substantial, exceptions are rare. 
Another cause of doubt would appear to be a not infrequent misunder- 
standing as to the nature of the law itself, which becomes rather easily 
misconstrued into a statement that the (stratigraphically) lower seams are 
of higher rank than the upper seams. Consequently the fact that a 
stratigraphically lower seam in one place is actually of lower rank than an 
upper seam elsewhere is mistakenly accepted as evidence against the rule. 
The value of Hilt’s law lies, first, in the fact that it is unambiguous 
in its significance, and, secondly, in the fact that since we know something 
of the increments of temperature and pressure which correspond _ to 
given differences of depth, and can compare these with the changes of 
rank produced, it gives us the data necessary to reverse the process, and 
to consider the rank of coals as an indication of the temperatures and 
pressures to which they have been subjected. , The temperature incre- 
ments are clearly so small that most geologists have been inclined to 
attribute the effects mainly to pressure. It is impossible to discuss this 
matter within the limits of this address, so it must suffice merely to refer 
to the experimental work, which has shown how readily coal, even in its 
present condition, yields volatile products at very moderate temperatures, 
and to the fact that in the effects of igneous intrusions we have a series 
of beautiful natural experiments in the alteration of rank of coals by heat 
alone. In some of these natural experiments the resultant effects on the 
coal appear to be in every respect analogous with the ordinary changes of 
rank; in others they are obviously different. In the latter instances it 
is not difficult to see that the results are due to the rapid application of 
somewhat considerable increments of temperature. ‘There appears to 
me to be quite good evidence to show, on the contrary, that in those cases, 
in which considerable changes of rank of the normal type have been pro- 
duced, the total increments of temperature have been quite small. 
