G41 
Judging from Mr. Mallet’s Record, most of the earthquake 
shocks here noted were not felt beyond the British Isles, and 
some were very local. And it is strange that the great earthquake 
ot Lisbon (Nov. 1st, 1755) does not appear to have produced 
much eftect in this country, for Mr. Mallet observes (p. 168) 
that “ in the British Isles actually sensible shocks were felt in 
but few places.” 
Connected with this subject of earthquakes and volcanoes is 
that of the upheaval or depression of the land to which I have 
alluded on a former occasion,* This upheaval is evident enough 
when wo contemplate the Norwich Crag, the Chalk, or any other 
deposit that must have been accumulated beneath the sea. 
In treating of the causes of these changes of level, Lyell, in 
his latest work, spoke vaguely, for lie says ; — 
“ Whether we ascribe the.se changes to the expansion of solid matter 
exposed to hydrothermal action, or to the melting of rock, or to the 
solidification of mineral masses, in whatever conjectures we indulge, we 
cannot doubt that, at some unknown depths the structure of the crust of 
our globe is gradually undergoing very important modifications.”+ 
Ihus wo have the change of material from one place to another, 
as in a volcanic eruption, and the filling in of cavities from 
Avliicli material had been ejected, by subsidence of the ground. 
Only in very recent years have the effects of de|JOsition and 
denudation been taken into account. The subject has been dealt 
Avith by the Rev. 0. Fisher, Avho concludes that the earth’s crust 
is about tAventy-fivo miles in thickness, and that it must bo in 
a condition of approximate hydrostatical equilibrium, so that any 
considerable addition of load Avill cause any region to sink, or 
any eonsiderable amount denuded off an area Avill cause it to 
lise. These AueAA's coincide A\'ith those e.xpressed on earth 
tremors, minute vibrations which appear to be influenced by 
barometric pressure. 
Subsidences of land in small tracts of ground arc produced in 
several Avays, both naturally and artificially. 
Naturally, they occur through the dissolution of Limestone 
* ‘Scenery of Norfolk,’ Trans. Norfolk and Norwich Nat. Soc. antea, 
p. 439. 
t ‘Principles of Geology,’ edit. xi. vol. ii. p. 197. 
I ‘ Physics of the Earth’s Cnist,’ p. 275. 
