from powerful shocks.
Where natural obelisks and steep taluses abound--features which would
have disappeared if the region had been moved by great shocks--we may
be sure that the field under inspection has for a great period been
exempt from powerful shaking. Judged by this standard, we may safely
say that the region occupied by the Appalachian Mountains has been
exempt from serious trouble. So, too, the section of the Cordilleras
lying to the east of what is commonly called the Great Basin, between
the Rocky Mountains and the Sierra Nevada, has also enjoyed a long
reign of peace. In glaciated countries the record is naturally less
clear than in those parts of the world which have been subjected to
long-continued, slow decay of the rocks. Nevertheless, in those fields
boulders are often found poised in position which they could not have
maintained if subjected to violent shaking. Judged by this evidence,
we may say that a large part of the northern section of this
continent, particularly the area about the Great Lakes, has been
exempt from considerable shocks since the glacier passed away.

The shores which are subject to the visitations of the great marine
waves, caused by earthquake shocks occurring beneath the bottom of the
neighbouring ocean, are so swept by those violent inundations that
they lose many features which are often found along coasts that have
been exempted from such visitations. Thus wherever we find extensive
and delicately moulded dunes, poised stones, or slender pinnacled
rocks along a coast, we may be sure that since these features were
formed the district has not been swept by these great waves.

[Illustration: Fig. 22.--Poised rocks indicating a long exemption from
strong earthquakes in the places where such features occur.]

Around the northern Atlantic we almost everywhere find the glacial
waste here and there accumulated near the margin of the sea in the
complicated sculptured outlines which are assumed by kame sands and
gravels. From a study of these features just above the level of high
tide, the writer has become convinced that the North Atlantic district
has long been exempt from the assaults of other waves than those which
are produced during heavy storms. At the present time the waves
formed by earthquakes appear to be of destructive violence only on the
west coast of South America, where they roll in from a region of the
Pacific lying to the south of the equator and a few hundred mile