MARYLAND GEOLOGICAL SURVEY 427

To give an intelligent and fair account of all the work done in
recent years in this special field of human activity would require far
more space and time than I have at my disposal. On the European
continent, in nearly every country, elaborate magnetic surveys are
either at present in progress or have just been finished or are in con-
templation. The most detailed one that has come to our knowledge
is that of Holland, by Dr. van Rijckevorsel, for the epoch 1891. 0,
embracing 278 stations over an area about equal to that of Maryland
or averaging about one station to every 40 square miles. This survey
of Holland was especially interesting from the fact that though it
was made over an area superficially destitute of striking geological
features, it nevertheless revealed marked disturbances. The author
sums up his conclusions thus: "Little even as we know about the
geology of the Netherlands, the magnetic maps must bring every one
to the conviction that in some cases, in. many perhaps, there must be
a direct relation between geology and terrestrial magnetism, and that
many of the magnetic features must be in some way determined by
the geological structure of the underground. What these geological
features might be we are at present unable to tell. What kinds of
rock may be 'hidden at a depth of 300 metres or more under the
peat bogs and heaths of the Netherlands, and the clay, sand and peb-
bles immediately underlying these, we do not know—rocks which,
although under ground, are yet perhaps in some places so near the
surface as to be an effective barrier against the inroads of the sea,
which has fair play in other districts. It is for geologists to tell us.
Magnetism will, in many cases, if carefully employed, be able to say
to geology: ' There is something hidden on this particular spot, it is
for you to tell us what it is. ' This does not mean that every mag-
netic feature must needs have its counterpart in a geological one, still
less the reverse. For we know perfectly that not every mineral is
magnetic, or capable of being magnetized, and therefore rocks of any
size may exist for which our needles show a supreme indifference.

netic Lines. —II. On the Accuracy of the Determination of the Local Dis-
turbing Magnetic Forces. —III. On the Relation between the Magnetic and
the Geological Constitution of Great Britain and Ireland.