y uplifting the
overlying mass, which it did not break through, into a dome-shaped
elevation. These side flows from dikes are termed laccolites, a word
which signifies the pool-like nature of the stony mass which they form
between the strata.

In many regions, where the earth has worn down so as to reveal the
zone of dikes which was formed at a great depth, the surface of the
country is fairly laced with these intrusions. Thus on Cape Ann, a
rocky isle on the east coast of Massachusetts, having an area of about
twenty square miles, the writer, with the assistance of his colleague,
Prof. R.S. Tarr, found about four hundred distinct dikes exhibited on
the shore line where the rocks had been swept bare by the waves. If
the census of these intrusions could have been extended over the whole
island, it would probably have appeared that the total number exceeded
five thousand. In other regions square miles can be found where the
dikes intercepted by the surface occupy an aggregate area greater than
that of the rocks into which they have been intruded.

Now and then, but rarely, the student of dikes finds one where the
bordering walls, in place of having the clean-cut appearance which
they usually exhibit, has its sides greatly worn away and much melted,
as if by the long-continued passage of the igneous fluid through the
crevice. Such dikes are usually very wide, and are probably the paths
through which lavas found their way to the surface of the earth,
pouring forth in a volcanic eruption. In some cases we can trace their
relation to ancient volcanic cones which have worn down in all their
part which were made up of incoherent materials, so that there remains
only the central pipe, which has been preserved from decay by the
coherent character of the lava which filled it.

The hypothesis that dikes are driven upward into strata by the
pressure of the beds which overlie materials hot and soft enough to be
put in motion when a fissure enters them, and that their movement
upward through the crevice is accounted for by this pressure, makes
certain features of these intrusions comprehensible. Seeing that very
long, slender dikes are found penetrating the rock, which could not
have had a high temperature, it becomes difficult to understand how
the lava could have maintained its fluidity; but on the supposition
that it was impelled forward by a strong pressure, and that the energy
thus transmitted through it was converted into hea