586 EEPORT— 1889. 



are preserved in the Britisli Museum froui the Kimmeridge Clay and Portland 

 Stone of Dorsetshire ; the former equal T. Heckeli, Thiolliere, from the French 

 lithographic stone, in size ; the latter are much smaller. 



6. Broioneichthys ornatus, gen. et sp. nov. Remains of a small elongated fish 

 discovered by Mr. Montagu Browne in the Low. Lias of Barrow-on-Soar, pertain to 

 a new generic and specific type, apparently related to the Belonorhynchidae. The 

 notochord is persistent and the neural and haemal arches are ossified, but there are 

 no well-developed ribs. The scales are thin, cycloidal, with prominent concentric 

 lines of growth, deeply overlapping and externally ornamented with ganoine 

 tubercles. Portions of a dorsal and ventral series of very large, narrow, pointed 

 ridge-scales are also observable. The cranial bones are invested with ganoine, and 

 are coarsely tuberculated. 



9. On the delations between the Geological Constitidion and the Magnetic 

 State of the British Ides. By Professors A. W. Rucker, F.B.8., and 

 T. B. Thorpe, F.B.S. 



During the last five summers the authors have determined the magnetic 

 elements at two hundred stations, distributed over the whole of the United King- 

 dom, and have employed the results of their observations in a study of the 

 disturbing magnetic forces which are in play in various districts. 



It is generally known that the magnet does not point due north, and that the 

 declination or deviation from true geographical north is different in diflerent parts 

 of the world. At London it is about 18° W., and is greater at stations which are 

 either to the north or west of the metropolis. 



Thus, if all disturbing causes were removed, an observer travelling due west 

 from London would find that the declination increased by about half a degree for 

 each degree of longitude. As a matter of fact the rate of increase, though equal 

 on the average to this amount, is irregular. Between London and Windsor it is 

 considerably larger, between Windsor and Reading smaller, than the mean. The 

 forces which produce these abnormal variations depend upon the geological 

 character of the district. They may be called local or regional distui-bing forces, 

 according as they influence small or large areas. 



The authors have calculated the direction and magnitude of these forces at all 

 the stations included in their survey, and have found that it is possible to divide 

 the British Isles into a comparatively small number of districts, in eacb of which 

 the disturbing forces tend towards certain definite points or lines. 



Two principal theories have been proposed to account for such phenomena. 

 Many igneous rocks, and notably basalt, contain magnetic oxide of iron, and the 

 deviations of the needle may be explained by the presence of such rocks, either 

 visible upon the surface or concealed beneath it. The other explanation associates 

 the deflections of the needle with disturbances of the earth currents of electricity, 

 produced by irregularities in the geological constitution of the country, and 

 especially by geological faults. 



In attempting to decide between these the authors point out (1) that even 

 igneous rocks, which are vary feebly magnetic when tested in the laboratory, 

 produce in large masses very considerable eflTects upon the magnet. Thus the 

 Malvern Hills, though composed of diorite in which magnetic polarity can barely 

 he detected, produce deviations of twenty minutes of arc at a distance of a mile 

 from their axis. (2) Mr. Preece, F.R S., chief of the Telegraph Engineering 

 Department of the Post Office, most kindly caused measurements of the earth 

 currents to be made in two districts in which the authors had discovered large 

 deviations of the magnet. In the neighbourhood of Melton Mowbray it was 

 foimd that the largest difference of potential per mile was about forty times less 

 than that which accompanies a similar deviation of the magnet at Greenwich 

 during a magnetic storm. In the neighbourhood of Reading and Windsor, though 

 the local deviations of the needle are such as only occur in very violent storms, no 

 measurable earth currents were detected. These observations are therefore incon- 

 sistent with a view which postulates surface earth currents as the cause of the 



