1360 EDWARD D. LYNTON 
evidence on both sides of the valley, but it is concealed by a deep cover of 
alluvium for several miles in the location chosen for an accurate deter- 
mination of the trace of this fault. The procedure was to run lines at 
intervals of 14 mile across the approximate location of the fault. The 
average length of the lines is approximately 1 mile. Magnetometer 
readings were recorded at each 200 feet. The course of the lines was N. 
27° E., approximately at right angles to the strike of the fault. 
As the work continued, it became increasingly evident that the 
Cretaceous area northeast of the fault showed much stronger vertical 
intensity than the Eocene on the southwest. Celluloid profiles showing 
vertical intensity plotted to scale were affixed to the map along the lines 
of survey, and the surface, outlined by the edges of these profiles, indicated 
a sort of magnetic plateau over the supposed Cretaceous area. As the 
valley floor is concealed beneath alluvium, a short line was run across 
the point of Lime Ridge where there are outcrops of rocks known to be 
Cretaceous, and high vertical magnetic intensities were found corres- 
ponding with those in the doubtful region. A sharp depression in the 
magnetic profiles near the assumed position of the fault was followed 
to the hills and proved to occur at the known location of the fault. This 
sharp depression in the magnetic profile has therefore been assumed to 
indicate the trace of the fault throughout the part covered by alluvium. 
The shape of the curve at this point is characteristically the same on all 
the profiles. On the basis of this indication the writer has drawn in the 
fault line, which in general lies close to that originally assumed, but has 
two offsets on the north which may have resulted from cross faults sup- 
posed to exist at this location, provided the assumptions here followed 
are correct. 
Several of the survey lines have crossed small faults, with much 
smaller indications. Seemingly there is no property of a fault as such 
that affects the magnetic instruments, and a fault will probably be in- 
dicated by them only where rocks of different magnetic properties have 
been brought opposite each other, or where a magnetic bed has been 
offset horizontally in crossing the fault. 
Magnetic marker beds.—An inspection of the magnetic profiles shows 
high points at intervals, which are found to be in definite lines. It is 
natural to assume that these lie over buried outcrops of beds.of stronger 
magnetic properties than those of adjacent rocks. In order to verify 
this conclusion some lines were run with observations at short intervals 
across exposed beds in line with these series of high points on the pro- 
files. It was found that some beds show very high values, which dis- 
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