The Magnetite Belt at Cranberry, JV. C. — Kiiiil>nll. 309 
the gossan. The reasons are not far to seek. First, as the 
course of the ore belt is alruost at right angles to the magnet- 
ic meridian terrestrial magnetism prevails over all but in- 
tense local attraction under exceptionally favorable or arti- 
ficial circumstances as above instanced. Second, as every 
separate lens acts as a single magnet the overlapping of lenses 
of unequal size, as well as overlapping of opposite terminal 
parts of separate lenses within the compass of the pyroxene 
belt, tends to neutralize their opposite polarity. Both posit- 
ive and negative polarity thus acting together produce no 
appreciable effect upon the short needles of simple instru- 
ments. Third, the disintegration of the ore lenses to a con- 
siderable depth, as commonly observed in this latitude, also 
tends to impair the polarity of the nearest original ore lenses, 
while lower and less decomposed parts of deposits are un- 
favorably influenced by the law of decrease of magnetic in- 
tensity in measure of the square of the distance. 
Besides such difficulties from the small measure of result- 
ant magnetic intensity, particularly in the case of complex 
development of ore lenses, the greatest difficulty of all op- 
posed to a detailed magnetic survey is from the densit}^ of the 
forest. And it is doubtful whether a systematic survey in 
spite of the obstacles instanced would afford satisfactorj^ re- 
sults as in the Archean iron fields of New York and New Jer- 
se}'' where glacial erosion has cleared the way for work of this 
kind by removal of the inert zone of rock decay. Instru- 
ments of precision would here be required like the field mag- 
netometer and transit, while as a preliminary a rectangular 
sectional survey would be indispensable in order to establish 
fixed points for magnetic observation and comparison. 
One of the more interesting occurrences connected with the 
ore belts is the development of homogeneous sub-crystalline 
pyroxene to a remarkable thickness. At the opening on the 
western ore belt above referred to this attains a thickness 
from 20 to 30 ft. In the eastern or Cranberry ore belt the 
thickness is much greater, though the pyroxene is less homo- 
geneous through weathering alteration. Whatever may have 
been the mode of accumulation of this material, the magne- 
tite associated with it api)ears to have been derived as well 
as the pyroxene itself by molecular separation or different!- 
