G39 
to similar causes, the subterranean forces must in these instances 
exhaust themselves in producing the earthquakes without giving 
rise to volcanic eruption. Professor J udd remarks that “ there can 
be no doubt that in the great majority of instances the forces 
originating earthquake vibrations and volcanic outbursts are the 
same, and independent lines of reasoning have conducted us to the 
conclusion that these forces operate at very moderate distances 
from the earth’s surface.”* The distances he mentions are from 
seven or eight to thirty miles. 
While several distinct causes may help to originate volcanic 
action, the actual eruption is now mainly attributed to the escape 
of imprisoned watery vapour and gases. Still wo have yet much 
to learn on this subject, for Professor A. II. Green says : “What it 
is that starts the wave wo do not exactly know. Put earthquakes 
always precede great volcanic eruptions, and in their case may bo 
reasonably supposed to be produced by the jar occasioned when 
imprisoned steam rends asunder a rocky barrier that has held it in. 
It is therefore not unlikely that some similar action originates all 
earthquakes. ”t 
Some earthquakes, however, may bo simply due to shrinkage and 
fracture of the earth’s crust, and of such disturbances we have 
abundant evidence in the joints, faults, and contortions seen in 
our rocks in many places, as on the Devonshire coast near Torquay 
and Clovelly ; but not in Norfolk, for there the contortions in 
the Drift are duo to glacial action. 
The oxidation of metallic substances in the interior of the 
earth is also admitted to be a likely source of local disturbance. 
The following is a list of all the earthquakes afiecting Norfolk 
of which I have been able to obtain a record, the principal 
authorities being given below, or cited with the record : — 
pRiKCiPAL Authorities. 
Rev. Francis Blomefield. ‘ An Essay towards a Topographical History 
of the County of Norfolk.’ 5 vols. (vols. 3 to 5 by the 
Rev. C. Parkin). Folio. Fersfield and Lynn. 
R. Mallet. ‘ The Earthquake Catalogue of the British Association, 
1S52 — 1858.’ Reprinted 1858. 
* ‘ Volcanoes ’ (1881), p. 344. 
t ‘ Physical Geology ’ (1882), p. 387. 
