56 ‘Dri G.. B.. Atlan on 
Some of the basaltic columns on the Dattenberg, after lying 
exposed for a long time to the action of the weather, deteri- 
orate to such an extent that a large column can be shattered 
into fragments with one blow of a hammer. 
The curves for Dattenberg bars Nos. 3, 4, and 5, shown in 
fig. 7, are similar to those for bars 1 and 2. Three bars 
were cut from a strip of rock taken lengthwise from the 
edge of a basaltic column. When in position bars 3 and 5 
lay near two adjacent and almost parallel faces of the column, 
which both showed signs of weathering; bar 4 was taken 
from between the other two. Whilst the initial susceptibility 
is different for each bar, being highest for the inside one, 
the temperature of maximum strength also varies, being 68°. 
48°, and 57° respectively for bars 3, 4, and 5. The tempe- 
rature of minimum permeability is not distinct, but all the 
bars show little sign of magnetization at about 250° C. 
‘These three curves were taken for the purpose of comparing 
the magnetic properties of bars taken from adjacent positions 
in the rock; and it holds for these bars and for Nos. 1 and 2, 
that the bar with the lowest temperature of maximum sus- 
ceptibility has also the highest initial susceptibility. 
Curves Nos. XIII. and XIV. are for bars Nos. 1 and 2 of 
Rowley dolerite. They each show feeble signs of a maximum 
at 109°, and another at 263°; but whilst the strength of bar 
No. 1 then decreases to a minimum at 445° followed by a 
maximum at 467°, magnetism disappearing almost entirely 
at 597°, bar No. 2 goes on increasing in strength, and has 
another maximum at 362°, reaching a minimum at a little 
over 600°. Bar No. 1 is unique in the series in this respect 
that no other exhibited so great a variation of magnetic 
strength as that shown in Curve XIII. between the tempe- 
ratures 380° and 520°, the susceptibility of the bar at 425° 
being almost half the value it had at 380° and 470°. During 
the cooling, bar No. 1 showed a maximum susceptibility at 
488°, whilst bar No. 2 was strongest at 446°. In their 
general character the two cooling curves are similar to those 
for Rowley Blue No. 2 (see Curves V., VI., and VII.). It 
may be noted tbat in the heating curves for Rowley Blue 
No. 2 (Curve IV.), and for Rowley dolerite No. 2 (Curve 
XIV.) which are very similar, and also in that for dolerite 
No. 1 (Curve XIII.), there is a brow in the dip of the curve 
corresponding to the period at which each bar has almost 
completely lost its magnetism. 
The last curve given is for a bar of magnetite from 
Magnet Cove, Arkansas, cut to the same pattern as the bars 
of basalt. This one was placed 3°3 cms, farther from the 
ao ’ 
Mitra. 
