198 Proceedings of Royal Society of Edinburgh. [sess. 
fathoms apart, are thrust to twenty fathoms apart if there happens 
to be ten fathoms of intrusive rock between them. A reference 
to almost any treatise on geology in which this relationship 
between intrusive masses and the “country rock” is discussed 
will at once prove that the view referred to has evidently been 
the one that the author had in mind. 
Amongst colliery people, who have to deal with these questions 
in a practical way, there has long been some difference of opinion 
upon this point ; some believing that trap rocks cut out the 
measures. But as they are “only practical men,” their opinion 
upon a geological matter is apt to be ignored. Furthermore, as 
will be evident from the sequel, many field geologists are now of 
opinion that intrusive masses usually replace the rocks they 
invade. 
It is obviously a matter of considerable commercial import- 
ance to test by field evidence whether the current view referred 
to above is or is not the correct one. This is especially the case in 
connection with the Scottish coal-fields, which are in many cases 
“much troubled with whin,” as the increasing demand for coal 
is leading to the prospecting of parts of coal-fields which have 
hitherto been left untouched, because the areas referred to have 
been known to be affected by intrusive masses. A little con- 
sideration will suffice to show that the question is one of at least 
equal interest to geologists, as it is one of wide-reaching import- 
ance, and as, moreover, it raises many questions in both chemistry 
and physics which are much more easily asked than answered. 
One may indeed go farther than even that, for if it can be shown 
that the current view is not in accordance with the facts, it is 
obvious that our views on the origin of eruptive rocks in general 
will have to be reconsidered, and we may even have to modify our 
opinions on some matters relating to the succession of events 
which took place in the earlier geological, or later astronomical, 
periods of the Earth’s history. 
Fully realising, therefore, the importance of the issues about 
to be raised, I shall endeavour, in the first part of this paper, to 
keep rigidly to a statement of the facts which bear upon this 
question, and then, after summarising the evidence, I shall go on 
to point out the conclusions to which the study of these facts 
