C.—GEOLOGY. 99 
difficulties, and all explanations hitherto proposed are so hopelessly in- 
adequate that we have sometimes felt compelled to doubt whether the 
facts really are as stated. But the phenomena have now been observed 
so carefully and in so many different districts that any real doubt as 
to the facts is out of the question, and we must still look for some 
adequate method by which the overthrusting could have been brought 
about. And if dozens of square miles of ground have been shifted over 
their foundations and away from their roots for many linear miles in the 
course of a single geological Period, who shall say what might not be 
accomplished in the course of Eras ? 
Important consequences flow from the fact that the goal and expression 
of most geological research is the construction of a map of the area studied. 
To the layman who studies a country with a geological map in hand, 
it is hard to resist the conclusion that the map is merely fanciful; he 
can see no evidence for the lines laid down or the symbols employed, 
and he is astonished when trenching or drilling proves their correctness at 
any particular point. Itis difficult for him to see or to realise the cumulative 
force of the aggregation of minute pieces of evidence, slight differences in 
slope or soil, variations in quality, quantity, or luxuriance of vegetation, 
variations in dryness or moisture, the distribution of culture, the extension 
into the area of some underlying tectonic plan—the laws of which may 
have been worked out elsewhere—and the thousand-and-one considerations 
which go to make up the mind of the geologist. 
It is, of course, perfectly true that the individuality of the surveyor 
enters not a little into the extrapolation of geological lines beyond the 
points where direct observation of the rocks is possible. So much is this 
the case, that it is feasible, from the inspection of his map, to gauge, not 
only the geological competence, experience, and attainments of the 
surveyor, but his knowledge and grasp of physiographic form, his power 
to see into intricate solid geometry, his artistic skill of hand and eye, and, 
above all, that indefinable quality, his ‘eye for a country,’ on which so very 
much depends. 
The construction of a map has the further advantage that it grows by 
the alternation of periods of observation in the field with periods devoted 
to the thinking out of structure after each day’s work and in the intervals 
between successive visits to the field, so that, with every return to the 
ground, the facts may be re-observed and lines re-tested in the light of 
growing knowledge. It is true that ideal observation should be so complete 
and exact that re-observation has nothing to teach; but, as a matter of 
fact, with a map as with a book, what one takes from it is what one brings 
to it, clarified, improved, and extended. There should be allowed to pro- 
fessional geological surveyors as much elasticity as possible, so that, in 
addition to detailed and exhaustive primary survey, there may be frequent 
revision in the light of their own work and that of their neighbours. In 
this respect the hand-coloured form in which geological maps were originally 
published has an advantage over the newer, cheaper, and more consistent 
colour-printed maps. 
Geologists should give a cordial weleome to the new aid provided by 
aerial survey and photography. This provides the last point of view of 
their areas which has been hitherto denied, though they have been in 
the habit of making use of the only substitute open to them, prospecting 
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