
GEOLOGICAL SURVEYING 299 
often happens in cases of inversion that this structure shows 
a combination of overturning and overthrusting. 
Folding and faulting of such extreme kinds are usually 
best developed in regions which have been subject to great 
deformation—regions the structural geology of which can 
hardly be unravelled by the tyro. The observer, who is 
prepared to work out complicated structures like those of 
N.W. Scotland, the Alps, etc, has got far beyond the need 
of an elementary hand-book. The beginner, however, who 
is anxious to become familiarised with the phenomena likely 
to be encountered in regions of complex structure, can hardly 
do better than study the beautiful maps of Wester Sutherland 
and Ross, which have been issued by the Geological Survey. 
With these maps before him, and with the help of the works 
mentioned in the footnote, the student will have some idea 
of the nature of rock-structures which are characteristic of 
“folded mountains.” * 
Eruptive Rocks.—The mapping of eruptive rocks is 
carried on in the same way as that of sedimentary strata. 
The outcrops of contemporaneous or effusive igneous rocks 
are not more difficult to follow than those of limestone or any 
other bedded rock. The boundary-lines of intrusive bosses, 
sills, and dykes, however, are more irregular, and, in the 
absence of sections, may sometimes be hard to trace. Rocks 
of this class, however, are usually more resistant than the 
rocks they traverse, and thus tend to project and form con- 
spicuous features at the surface. In mountainous regions 
where the rocks are generally well exposed, the field-geologist 
is more likely to be troubled with the abundance than with 
the paucity of the evidence. In the case of a mass of granite, 
for example, the junction-line is apt to be very irregular— 
larger and smaller veins penetrating the adjacent rocks in all 
Ginections. Lhe details of ‘structure are often, indeed, so 
intricate that the most the observer can do is to generalise 
* The sheets of the 1-inch map are as follows :—81, 91, IOI, 107, I14. 
Consult Annual Reports of the Geological Survey, 1892-96 inclusive ; 
Summary of Progress of 1.M. Geological Survey for 1897. See also 
Quarterly Journal of the Geological Society, vols. xliv. 378 ; xlviil. 22 
1. 661. The whole region is described in great detail in the Geological 
Survey’s Memoir—Z7he Geological Structure of the North-West High- 
lands of Scotland, 1907. 



