58 Dr. G. E. Allan on 
Dattenberg specimens, it being difficult to find any difference 
between ilie heated and the unheated sections when examined 
either by eye or under the microscope. The redness which 
was so prominent in the other heated sections was wanting 
in that of the Dattenberg basalt. This agrees with what was 
found in regard to its magnetic behaviour, viz. that heating 
produces less permanent change in the magnetic properties 
of Dattenberg basalt than in (bee of Rowley rock. »Hig.ts 
is from a thin negative and the effect of heat on the doch 
is exaggerated. The photomicrographs, for which [I am 
ieee oned! to Mr. J. D. Whittles, are taken with a magnification 
of 40 diameters. 
Susceptibility of Basalt. 
The magnetometric method, when employed to test bars 
of the size used in the foregoing experiments, does not lead 
to accurate values of the susceptibility. The results below 
are therefore to be taken as giving a comparison of mag- 
netic intensities of the different specimens rather than as a 
series of accurate absolute values of the susceptibility. 
The bars employed, and their dimensions, are given in 
the following table. 







Bar. Weight. | Density. | Length. | Volume. 
Rowley Blue. | gms. cms. e.c. 
INO. Ueber ee | 63054 2°895 10-4 21°77 
ING See std 63°437 2°864 10°4 22°14 
Rowley Dolerite : 
INO LT eet ence 59°261 227 NO 2 i ee 
NOW ES seh 58642 2715 10-4 21:46 
| Dattenberg. | | | 
| UN Oe Mer meiatswiats | 73°800 3006 10-4 24°56 
IN@r ees esttas.c cee | 663 2°99 LOAN) ASS Bae 
INO i Scckg .cssens | 67-010 2956 |. 104 22°68 
CR ae eee i) OO, S024 | WO oa 2a 
INOiD.. sored | 69:706 |  3:006 104 23°19 
Magnetite ............, 11749 |  4°628 10-4 25°39 



| 
| 

To determine the susceptibilities of the rocks, a constant 
was found, from experiments on two bars, which, when mul- 
tiplied by the deflexion produced by any bar, gave the 
magnetic moment of the latter. For example, the bar 
Rowley dolerite No. 2 was placed in the magnetizing coil, 
with its centre 27°8 cms. from the magnetometer-needle. 
By reversing a field of 46 c.a.s. the bar produced a deflexion 
of 32°2 mms. on a scale at a distance of 175°5 cms. The 
