s have been
mainly accumulated by shellfish, their average thickness is less than
half a foot.

When we examine the partitions between the layers of limestone, we
commonly find that, however thin, they generally extend for an
indefinite distance in every direction. The writer has traced some of
these for miles; never, indeed, has he been able to find where they
disappeared. This fact makes it clear that the destruction which took
place at the stage where these partings were formed was widespread; so
far as it was due to earthquake shocks, we may fairly believe that in
many cases it occurred over areas which were to be measured by tens of
thousands of square miles. Indeed, from what we know of earthquake
shocks, it seems likely that the devastation may at times have
affected millions of square miles.

Another class of accidents connected with earthquakes may also
suddenly disturb the mud on the sea bottom. When, as elsewhere noted,
a shock originates beneath the sea, the effect is suddenly to elevate
the water over the seat of the jarring and the regions thereabouts to
the height of some feet. This elevation quickly takes the shape of a
ringlike wave, which rolls off in every direction from its point of
origin. Where the sea is deep, the effect of this wave on the bottom
may be but slight; but as the undulation attains shallower water, and
in proportion to the shoaling, the front of the surge is retarded in
its advance by the friction of the bottom, while the rear part, being
in deeper water, crowds upon the advancing line. The action is
precisely that which has been described as occurring in wind-made
waves as they approach the beach; but in this last-named group of
undulations, because of the great width of the swell, the effect of
the shallowing is evident in much deeper water. It is likely that at
the depth of a thousand feet the passing of one of these vast surges
born of earthquakes may so stir the mud of the sea floor as to bring
about a widespread destruction of life, and thus give rise to many of
the partitions between strata.

If we examine with the microscope the fine-grained silts which make up
the shaly layers between limestones, we find the materials to be
mostly of inorganic origin. It is hard to trace the origin of the
mineral matter which it contains; some of the fragments are likely to
prove of Volcanic origin; others, bits of dust from meteorites; yet
others, dust blown from the land, which may, as