NAGGR LE 
465 
THURSDAY, SEPTEMBER 16, 1886 
EARTHQUAKE DISTRIBUTION 
Alphabetical Catalogue of the Earthquakes recorded as 
having occurred tn Europe and adjacent Countries 
arranged to serve as a basts for an Earthquake Map 
of Europe. By Joseph P. O’Reilly, C.E., Professor of 
Mining and Mineralogy, Royal College of Science, 
Dublin. Transactions of the Royal Irish Academy, 
Vol. XXVIII. “ Sczence.” (Dublin, 1886.) 
HE distribution of earthquakes may be studied either 
geographically or topographically—either along its 
broad lines in connection with the general physics of the 
globe, or in its smaller details with immediate and minute 
reference to local peculiarities. The field of inquiry is in 
both directions vast and comparatively unexplored. As 
regards what we may call circumstantial seismography, 
little has been done in the particular application of gene- 
ral principles to explain the apparent caprices of disturb- 
ances. These are innumerable and fantastic; yet, by 
patient observation, they can to a certain extent be brought 
within the range of strict physical reasoning. The laws 
of wave-motion, taken in connection with the facts of 
geological formation, will account for a good deal. Not, 
however, for all. Subterranean agencies introduce an 
element of uncertainty into the calculation which no 
diligence or ingenuity avails wholly to eliminate. Thus, 
in South America certain districts are observed to remain 
year after year, perhaps century after century, unscathed 
amid surrounding devastation. They are hence called 
“bridges,” the disturbance seeming, as it were, to flow 
beneath them, like a river under a bridge. Prof. Milne 
suggests that their immunity may be due to the total 
reflection of earth-vibrations which would otherwise reach 
them ( Earthquakes,” p. 141). An obstacle to the pro- 
pagation of such is besides often interposed by faults and 
fissures. The partial repose of Quito is attributed to the 
frequency of cafions in its vicinity; as, similarly, the 
Capitol of Rome and the citadel of Capua were said to 
be protected by numerous deep wells or springs sunk 
round them. 
“ Bridges,” however, cannot invariably be depended 
upon. Their privilege of safety is liable at any moment 
to be withdrawn. The peninsula of Caraya, for instance, 
forming the northern shore of the Gulf of Cariaco, was 
never known to be shaken until December 14, 1797 ; yet 
it has since had its full share of disturbance. The inte- 
rior arrangement of strata was here doubtless subverted, 
and the barrier to the extension of shocks from the chalk- 
hills of the mainland to the slate-rocks of the opposite 
peninsula overthrown, by the violence of the blow which 
destroyed Cumana. Centres of disturbance, too, shift 
and travel in a way to defy anticipation ; and the effects 
of the interference of earth-waves, to which some terrific 
catastrophes, as well as many apparent anomalies of rest 
or commotion, are ascribed, can rarely be calculated 
beforehand. Nevertheless, much knowledge of high 
value, both practical and theoretical, might be acquired 
by the continuous topographical study of earthquakes in 
countries exposed to their ravages. Prof. Milne states 
that in Japan (where, through his initiative, more has 
VOL. XXXIV.—No. 881 
been done in this branch than in any other part of the 
world), although earthquakes occur there at the average 
rate of two a day, yet with proper care a building site 
may be chosen as free from shocks as if it were situated 
in Great Britain. 
The publication now under review is, however, directed 
to a different and a wider purpose. Prof, O’Reilly had 
already compiled a “Catalogue of Earthquakes” for 
these islands (see NATURE, vol. xxxi. p. 351) ; he has now 
accomplished a similar task for the whole of Europe with 
the outlying districts along the Mediterranean and Black 
Sea shores, designed to form the basis of a map showing 
the larger geognostic relations of these phenomena. 
Without some such picture, he rightly observes, no geo- 
logical map should be regarded as complete. The latter 
displays results ; the former represents the forces at work 
to produce them. The importance of earthquakes in 
geological history is great and far-reaching. Their con- 
nection with the physical structure of a country is most 
intimate. Every one of its leading features is related to 
them, either causally or consequentially. The main lines 
of jointing and fissuring, with the inseparably associated 
strike of coast-lines, are, according to a view originated 
by our author, directly conditioned by the prevailing 
direction of earthquake shocks. Its correctness can be 
tested only by statistical inquiries such as those of which 
he here gives us a laborious example. Other questions 
of interest awaiting similar elucidation relate to seismic 
action with reference to coal-fields and to the progress of 
elevation or subsidence. 
Fuchs gives many instances of shocks limited to or 
originating in carboniferous districts (‘‘ Vulkane und Erd- 
beben,” p. 196), and explains them by the progressive 
decomposition of organic matters quickened by the 
admission of air in working the shafts. The resulting 
escape of fire-damp diminishes the volume of the beds; 
they give way with a concussion, and an earthquake 
ensues. Extremely curious, in this connection, is the 
close agreement between the curves denoting the monthly 
frequency of earthquakes and of colliery explosions 
pointed out by Prof. O’Reilly (Zvavs. R. Irish Acad2my, 
vol. xxviii. p. 297). Each shows a strong and precisely 
coincident maximum in March, while the earthquake 
maximum in November is less perfectly matched by a 
conspicuous increase, one month later, in the number of 
explosions. The analogy of the equinoctial maxima of 
aurore and magnetic disturbances cannot fail to suggest 
itself; and there are other indications that seismic and 
magnetic perturbations are not wholly extraneous to each 
other. Both auroree and earthquakes, for instance, dis- 
tinctly gain in frequency during the half-year which in- 
cludes our winter season and the perihelion passage of 
the earth; and there have been too many and too close 
coincidences between their occurrences to be purely 
accidental. 
Sudden changes of level, especially depressions, are an 
ordinary concomitant of earthquakes. An internal col- 
lapse of the strata in some cases produces the shock; in 
others the shock ruptures supports or overturns founda- 
tions long unsound. Degradation by water has perhaps 
undermined them ; contractions have taken place through 
cooling, through chemical action, possibly through slow 
evaporation. At last a crash comes, and a tract of land, 
x 
