METHODS OF CORRELATION 1130
which are undoubtedly due to eustatic movements of the sea, be-
comes more and more apparent. The widespread mid-]Jurassic
transgression of the sea over Europe is well known and the discon-
formity (and occasional unconformity) produced by this trans-
gression has been used for widespread correlation. The great
Mid-Ordovicic hiatus first observed in North America. (Grabau-
16; 18), and the similarly widespread hiatus in the Upper Ordo-
vicic (Weller-52), are now known to be marked in North Europe
as well (Bassler-5). In like manner the Mid-Siluric hiatus and
disconformity so widespread in North America appear also to be
present in the Baltic region of Russia and Sweden and in the
Bohemian Paleozoic district. The probabilities are that in Mid-
Siluric time the sea left a large part of the present land area dry.
Similar widespread disconformities are recognized in the Mesozoic.
6. Correlation by regional metamorphism.
Regional metamorphism has already been defined as an altera-
tion or metamorphism, which affects extensive regions and which is
primarily due to tectonic disturbances. Such metamorphism may,
of course, occur at any time in the history of the earth, but when-
ever it does occur it will affect all the formations of the region in
which it takes place, though, obviously, some formations may be
more strongly affected than others. This being the case, it follows
that, wherever unaltered rocks overlie the metamorphosed ones,
the age of the former cannot date back of the period of meta-
morphism, and that, hence, the lower limit of their age is fixed
by this period of metamorphism. Evidently there is no guarantee
here, however, that the strata of the overlying series are all of
the same age, though within moderate limits this is probably true.
One general rule may, perhaps, be formulated, and within certain
limits applied, namely, that, of two formations in contact, the more
strongly metamorphosed one is the older. Here, however, the same
caution is necessary that is required in applying the rule of greater
deformation to two deformed formations in contact. Some forma-
tions are more subject to metamorphism than others, just as some
formations are more subject to deformation.
The method of correlation by metamorphism 1is, perhaps, the
most applicable to the determination of the boundary line between
the pre-Paleozoic and later formations, though even here it seems
not always to be reliable. This would appear from the fact that
extensive sedimentary formations, such as the Belt series of Mon-
