O'Reilly— (9?^ Lines of Jointing of Bray Head^ S^^c. 231 
determine with care, and systematically, the various directions of 
jointings and dislocation manifest on the surface of the country. 
Were this done, then, taking for granted that the greater number, at 
least, of the principal lines of fissuring and jointing may be ascribed to 
seismic action, and that they resulted from more or less violent and 
repeated seismic undulations, proceeding from either volcanic or seismic 
centres, the positions of these would lie on lines normal to the lines or 
systems of fissuring and jointing, that is, on the directions of the un- 
dulations, and would, to a certain extent, be further defined by the 
dips of the observed joints or fissures. Moreover, some idea or esti- 
mate of the intensities of the earthquakes might be considered as being 
furnished by the characteristics of the observed jointings or fissurings, 
and in this way the relative importance of the centres of action be defined, 
l^or is it only from the point of view of geological structure that the 
precise determination of the various systems of jointing of a country 
and their related centres of action are of importance. Por the study 
of terrestrial magnetism such determinations would seem to be of still 
greater scientific interest, since, in a very remarkable study of the 
Phenomena of Terrestrial Magnetism, by Dr. Edmund Neumann {Die 
Ersclieinungen des, Erdmagnetismus in Hirer Ahhd7igigheit vom Ban der 
JErde, Stuttgard, 1887), he shows, by a map of Japan, on which he has 
laid down the directions of its isogenic lines of magnetism, the inti- 
mate relation apparent between the great lines of fissuring of that 
island and the magnetic variations which he had therein observed. 
Indeed, the main object of his Paper is to bring out distinctly this 
remarkable connexion. He says, p. 19, "The isogone of 5° W. cor- 
responds in its general course with a line of structure. (Die Isogone 
von 5° W. fallt in ihrem allegemeine Yerlauf mit einer tektonischen 
Linie zusammen.) This," he adds, "is a result of the very highest 
importance. We may say," he adds, "that the entire mountain chain 
describes at the points where occurs the 'fossa magna' — a crack or 
bend — the isogones corresponding to this crack or bend" — (Wir konnen 
sagen, das ganze Gebirge, beschreibe, dort wo die fossa magna liegt, 
eine knickung ; die isogonen machen diese knickung mit.)^ It would, 
^ In the same page lie thus describes the fossa magna: — Ich glaube jetzt wie 
vormals behaupten zu diirfen, dass die fossa magna eine Spalte, eine Zerreissung 
darstellt, keinen Jungendlichen Einbruch eine 8palt sogar, die von hohein Alter 
ist wenn auch jiinger als die longitudinale Hauptbruch des ganzein Gebirges. 
"I think 1 may assert now as formerly that the 'fossa magna' represents a 
great joint or fracture — no mere recent break, but rather, indeed, a joint wliich is 
of great age, even though more recent than the longitudinal main fault of the entire 
mountain." 
