62 ° SECTIONAL ADDRESSES 
If it be true that the rank of coals has been determined mainly by the 
depths to which they have been buried, then it is natural to look for some 
relation between the varying rank of any seam and its present depth 
below the surface, or to relate it to axes of folding. Much has been 
written on this subject, and completely contradictory conclusions reached. 
But this is to be expected. The coal can only indicate the maximum 
temperature or pressure to which it has been subjected, since change of 
rank is almost certainly an irreversible process ; decrease of temperature 
and pressure will not restore it to its former level. Now a little reflection 
will show that, after the completion of the coal-bearing series, the most 
general result of subsequent folding will be elevation and denudation, 
leading to a decrease rather than an increase of load. In so far as this is 
true, the folding will not be reflected by any variation of rank; while, on 
the contrary, any circumstance which does lead to a further increment of 
temperature will leave its mark. Later burial of the whole series below 
an unconformable cover may lead to a change of rank in the deeper parts 
of the seams, while leaving the upper portions untouched. The time 
factor, moreover, can by no means be left out of account. Geo-isotherms 
creep with exceeding slowness, even in terms of the rate of sedimentation 
and denudation. The duration of burial as well as its depth is therefore 
material. In fact, the whole sequence of events which have determined 
the maximum temperature and pressure reached at any point must have 
been much too complicated to be readily decipherable from the present 
disposition of the rocks. 
In all these complications, however, one cardinal principle remains. 
At any given place, both temperature and pressure must always have 
increased downwards—apart, of course, from the influence of igneous 
intrusions. And so, while the interpretation of the lateral variation of 
rank is involved in many complicating factors, Hilt’s law remains as a 
simple and significant sign. Even in this case the significance is liable to 
be obscured by the variation of the original coal substance of the different 
seams; but the technique of the microscopic study of coal has now 
reached the point at which, I believe, the effect of this factor can be almost 
completely assessed and eliminated. If this be so, then we are in a position 
to use coal as a geological thermometer—or, perhaps, combined thermo- 
meter and barometer—and we may set about calibrating it by means of 
a thorough study of Hilt’s law and of the effects of igneous intrusions. 
But we must never forget that the thermometer has one peculiarity—it is 
a maximum thermometer only. 
