MARYLAND GEOLOGICAL SURVEY 493

lines of equal magnetic declination (variation) run very irregularly
over the region embraced by the Piedmont Plateau (central and north-
eastern Maryland), while over the Coastal Plain (southern and south-
eastern Maryland) they are fairly regular. The counties that are
especially disturbed are Cecil, Harford, Baltimore, Carroll, Howard,
Montgomery and Frederick. In the regions covered by these coun-
ties the density of the stations must be greatly increased before we
can be sure of giving a true representation of the distribution of the
earth's magnetism as manifested by the declination.¹ Additional
observations made since the drawing of these lines, especially in Har-
ford County, clearly demonstrate that little dependence can be put
upon lines constructed from a small number of stations. The line 6°
W. as at present sketched must be regarded as but a rough approxima-
tion to the truth. A more detailed investigation in this region will
result in a number of closed areas, where the values will be smaller or
larger than those obtained by direct interpolation. Thus east of line
6° W. there are a number of places where the declination drops down
to 5i° and less, whereas the values should have been greater. For
example, Elkton gives but 5°.4 for 1900. The line 6° W., as at
present projected, is doubtless not far from the position which the line
would have were it not for the marked disturbances referred to. Its
general direction harmonizes with that as indicated by the distant
observations in Pennsylvania.

The same marked anomalies in the distribution of the earth's mag-
netism have been revealed by the other preliminary magnetic map,
viz., the map giving the lines of equal magnetic inclination and the
(Plate XVI) lines of equal horizontal magnetic force. These pre-
liminary maps have served a useful purpose in guiding me in the map-

¹ The results thus far obtained from the work of the present year clearly
indicate that the curves presented in this report have not exaggerated the
amount of disturbance in the distribution of the earth's magnetism over
the counties enumerated. Thus, for example, the declinations observed at
Linden and Rockville—both in Montgomery county and distant from each
other seven miles—differ from each other by nearly two degrees. At Linden
the declination for January 1st, 1900, will be 3°.6 and for Rockville 5°.5. It
will be seen that the isogonic line (4 1/2°)—drawn before the Rockville obser-
vations were made—would represent a mean of the two stations for this
locality quite satisfactorily.