Dr, E. Nawmann—Magnetism and Earth Structure. 535 
beach. -On the east coast this depression is, so far as my memory 
goes, at least seven metres high, and lies to the side of the Forcina 
dyke. It seems dependent upon the clay through which the dyke 
was injected being easily subject to erosion. This deposit looks much 
like a Pliocene clay, but my hurried and forced departure from the 
island prevented me from more fully investigating it, as also the actual 
height above sea-level of the lowest edge of the breach. Dolomieu, 
who visited the Ponza Islands in March of 1786,! says: “Cette Ile 
(Palmarole) est divisée en deux parties presque égales par un canal 
étroit qui la traverse vers la moitié de sa longueur, et dans lequel on 
passe en barque.” Besides this clear statement he gives a map of 
the island showing the canal in the position of the amphitheatre 
depression, and the lowest breach in the east coast-line. At 
present the lowest part of this depression I noticed strewed with 
boulders covered with Serpule and other marine growths, looking 
quite fresh, but above sea-level. We have evidence here of a very 
striking kind, of a very considerable elevation during a period of 
little less than a century—as important I believe as any recorded. 
I regret that I did not make more detailed observations, but the un- 
fortunate circumstances which caused my departure, together with the 
fact that it was only after my return that on looking at Dolomieu’s 
map, the fact struck me. I hope, however, to carefully investigate the 
matter next summer, so as to make public such an important piece 
of geodynamical evidence. Of course we have other prehistoric 
evidences of elevation in abundance in these islands, and one in 
particular that struck me, namely, the occurrence of beds of well- 
rounded pebbles half-way up the cliff of Monte della Guardia at 
Ponza. But the elevation of an island from 5 to 10 metres or more 
in a century would be of inestimable value as a cardinal fact in 
geology, if fully confirmed. 
I].—Terrrestriat Magnetism as Mopirrep BY THE STRUCTURE OF 
THE Harru’s Crust, anD Proposats Concernine 4 MAGNETIC 
SURVEY OF THE GLOBE. 
By Dr. Epmunp NauMANN. ; 
(Concluded from the November Number, p. 490.) 
[PLATES XV.-X1X.] 
| WILL now put forward some details, for it seems necessary 
that some proofs of what has been maintained in general 
should be given, and 1 shall therefore quote some examples of the 
intimate connexion between earth-structure and terrestrial magnetism. 
No diagrams could indicate the relations better than Locke’s Magnetic 
Sections across the Hudson, which were taken at three different 
points of the Palisades, Snake Hill, Fort Lee and Patterson (see 
Plate XV.). The dip and intensity were in each case determined at 
a number of points along a line perpendicular to the range of cliffs, 
1 Jles Ponces, 1788, p. 128. 
