536 Dr. E. Naumann—WVagnetism and Earth Structure. 
and the curves rise very suddenly to maxima just above the edges 
of the cliffs, which are composed of columnar diorite. This diorite 
is widely distributed and lies between masses of Triassic sandstone ; 
the dip is slight and towards the west. On the east side of the cliffs 
Laurentian gneisses are exposed. The close proximity of formations 
so far apart in age must be caused by a great fault, and the perpendi- 
cular dislocation amounts at least to some thousands of feet. In 
Locke’s opinion the very sudden rise and fall of the magnetic curves 
are due to the columns of diorite being magnetized by terrestrial 
induction, but I think there can be no doubt that the great fissure 
is the chief cause of the sudden changes. 
Another beautiful example is afforded by Nipher’s “ Magnetic 
Survey of the State of Missouri.”* The isogonic line of 7° 30! 
describes a loop, in the centre of which lies the Pilot Knob, a hill 
662 ft. high, and composed of porphyry, porphyry conglomerate, and 
layers of hard red sandstone. To the east is the knob connected 
with other porphyry hills. If we remember that the Pilot Knob can 
be nothing but an old volcano, whose subterranean core passes to 
great depths, we can well imagine how the hearthlike eruption may 
deflect electric currents from their direct track, and cause irregu- 
larities in the magnetic elements. 
Marvellous coincidences between magnetic curves and lines of 
great geological disturbance are met with in Asia. It seems as 
if the Himalayan range exerted some controlling influence over 
the isogonic lines; some of these follow the direction of the 
mountain chains, and the way in which the 2° 30’ and 38° curves 
go high up the Brahmaputra valley and then turn back to the 
S.W. is very striking (see Plate XVI.). Rijckevorsel, the well- 
known surveyor of the Indian Archipelago, speaking on this 
subject, says, that “It is as if the land had some power of coercion 
over the isogonic lines.”? J venture to submit that it is not the _ 
land which exerts this influence, but the great longitudinal ruptures 
indicated by the submerged mountain range. It may be that in 
a great portion of Asia the isogones coincide with lines of great 
geological displacement. 
Another illustration in support of our theory may be taken in 
Europe. The district between the Carpathian Mountains and the 
Transylvanian “ Erzgebirge” forms an almost circular basin; this 
region was surveyed by Kreil about forty years ago, and by Schenzel 
in 1875-76. During the interval the isogones have (of course) 
changed their position, but still the two maps, one for the period 
' Nipher, F., Magnetic Survey of Missouri. Fifth Annual Report, Trans. St. 
Louis Ac., vol. iv. No. 8, p. 516. Chart of the Magnetic Variation in Missouri, St. 
Louis, 1880. Sold at Robert Beneck’s, St. Louis. Chart of the Magnetic Variation 
in Missouri (photograph of a plaster chart), St. Louis, 1881. 
? Rijckevorsel, Report to His Excellency the Minister of the Colonies on a 
Megnee Survey of the Indian Archipelago, made in the years 1874-1877. Amster- 
am, 1879. 
$ Schenzel, Beitrage zur Kenntniss der magnetischen Verhaltnisse im Sudostlichen 
Ungarn, Repert. f. Experimental Physik, f. physikal Technik, f. math. u. astron. 
Instrumentenkunde, Herausgeg. vy. Carl, Miinchen, 1877, Bd. XIII. s. 165. 
