the Magnetism of Basalt. 57 
magnetometer needle than the basalt bars, and the field 
employed was 3°6 ¢.G.8., or 1/13 of the field used to mag- 
netize the basalt. The curve for ascending temperature 
is somewhat irregular, and there appears to be a maximum 
at or near 280°, and also at 475°, just before the magnetite 
begins to lose magnetism. During both the heating and the 
cooling the thermometer indicated a sudden change of tem- 
perature at 552°. In magnetite, therefore, the magnetic 
transformation is accompanied, as in iron, by a sudden 
change of temperature. The value 552° and the general 
>) e 
form of the heating curve are in good agreement with the 
results obtained by Siicse Barton and Williams, who give, 
in the volume mentioned*, the value 557° for the temperature 
of the minimum and also a curve, drawn to an arbitrary 
scale of permeability, which, however, shows no maximum 
at the higher temperature. Heating has the effect of con- 
sider ably reducing the susceptibility of the magnetite, and 
during the cooling a maximum was reached at 344° C. The 
magnetite, when withdrawn from the fur nace, was coated in 
places with the same red oxide (?) as the bars of basalt, the 
coating being present chiefly at one spot where a part of 
the original exposed surface of the magnetite still remained. 
Microscopic examination of the rocks was next proceeded 
with with a view to elucidating the effect of heat on the bars. 
Sections of the three different kinds of rock having been 
procured, the chips from which these sections had been taken 
were placed in the furnace and raised to 700°-800° for an 
hour. Sections of the heated chips were also obtained, and 
the photomicrographs given in figs. 10-15 (PI. II.) show parts 
of the sections, figs. 10, 12, and 14 being unheated rock, 
and figs. 11, 13, and 15 heated rock. No change could be 
oO . 
detected in the chief magnetic constituents of the rocks, viz. 
magnetite in the Dattenberg basalt, and ilmenite in the 
Rowley rocks, the magnetite being easily distinguished in 
fig. 14 as black opaque oranules, often in well-formed 
crystals, scattered uniformly throughout the section, whilst 
the ilmenite appears in fig. 10 as larger black masses, and in 
fig. 12 in the form of long black needles and plates. The 
observed change had taken place in the alteration products 
of the rocks, such as chlorite and serpentine. The general 
uppearance after heating indicated a cracking of the er rystals, 
the cracks being filled with the same reddish powder as had 
appeared on the outside of the bars. This powder being 
opaque the micrographs of the heated rocks appear darker 
in consequence. Heat appeared to have little effect on the 
* PB. A. Report, Edinburgh, 1892. 
