
GEOLOGICAL SURVEYING 303 
mapping slates, therefore, the chief danger to be avoided is the mistaking 
of cleavage for bedding. It is necessary, however, always to note the 
direction of the strike and dip of the cleavage-planes, especially when 
the bedding is obscure or obliterated. For the strike of the cleavage 
coincides more or less closely with the axes of folds and plications, and 
is thus helpful in unravelling the geological structure of a complicated 
region. . 
Regional Metamorphism.—Not much need be said on the subject of 
mapping an area in which regional metamorphism has been developed— 
the structural geology being frequently highly complicated and obscure, 
and hardly to be attempted by one who is not well versed in petrography, 
and has had little experience in geological surveying. Nevertheless, 
even a beginner will find much to interest him in trying to puzzle out the 
structure of such a region. 
We have already learned that regional metamorphism is not in- 
frequently a result of deformation. In other words, the rocks of sucha 
region have been more or less compressed and buckled up or folded, and 
in many places have yielded to tangential squeezing and crushing, 
whereby overthrusts on a grand scale have often been effected. In 
mapping an area which exhibits such phenomena, it is obviously most 
important that we should be able to lay down the axial lines of the chief 
folds, and the position of all considerable thrust-planes. This may be 
done without troubling ourselves at first as to purely theoretical questions 
concerning the particular chemical and mineralogical changes through 
which the rocks may have passed. It is more than likely, however, that 
as we proceed with our field observations we shall be confronted with 
evidence that may go a long way to show not only what the original 
character of the rocks may have been, but to reveal the successive 
changes which some of them have undergone. 
Bearing in mind, then, that the rocks, whatever their original character 
may have been, are arranged in folds, we shall expect to find the position 
of these indicated by the outcrops of more or less persistent zones or 
belts of different kinds of schistose rocks, all having approximately the 
same trend. These bands, we may safely assume, represent the general 
strike of the series. Not infrequently, however, we may traverse wide 
areas throughout which only a monotonous succession of one and the 
same kind of rock may appear. Nevertheless, if our traverses be 
sufficiently extensive, we may expect ere long to meet with other types 
of rock, the relative position of which will enable us to determine the 
general strike or alinement of the whole complex. The observer must 
be on the constant outlook for bands of rock which are characterised 
by the presence of minerals peculiar to themselves. Knowing that the 
production of these minerals is due in all probability to some peculiarity 
in the composition or constitution of the original rocks, their presence 
may sometimes be as useful in working out a stratigraphical succession 
as the occurrence of fossils in a series of unaltered strata. Beds and 
bands of ores not infrequently occur in connection with particular kinds 
of schist, and have in certain regions, as in Norway, been followed 
