Reports and Proceedings—Geological Society of London. 3877 
rates of propagation cannot be compared, owing to the time of 
occurrence of the Colombian earthquake being unknown; but the 
interval between the arrival of the first and second phases is found to 
be longer in the case of this earthquake, by an amount much in excess 
of any probable error of record or interpretation. This ditference 
indicates that the rate of propagation of the second-phase waves was 
relatively slower in the case of the Colombian earthquake, and, 
consequently, a difference in the constitution of the matter through 
which they were propagated. The Japanese records of the San 
Francisco earthquake also give an interval between the first and 
second phases which is ereater than the average, the wave-paths in 
this case too being oceanic. 
From these facts the general conclusion is drawn that oceans and 
continents are not mere surface irregularities of the earth’s form, 
but are accompanied by, and probably related to, differences in the 
constitution of the earth beneath them, which extend to a depth of 
about one-quarter of the radius. It is not possible to state exactly 
in what this difference consists, beyond that it causes the rate of 
propagation of the second-phase wayes to be’ less, in comparison with 
that of the first-phase waves, under the oceans than under the 
continents. 5 
5. ‘*The Swansea Harthquake of June 27th, 1906.” By Charles 
Davison, Sc.D., F.G.S. 
With the exception of the Hereford earthquake of 1896, the Swansea 
earthquake was the strongest which has been felt in this country for 
more than 20 years. It ‘disturbed an area of 66,700 square miles, 
reaching from Rochdale on the north to Penzance on the south, and 
from beyond } Maidenhead on the east to Waterford on the west. The 
centre of isoseismal 8 lies about 3 miles west of Swansea, the longer 
axis of the curve being directed E. 5° N. and W. 5° S. At Swansea, 
Neath, etc., the total “number of chimneys saa down or damaged 
must amount to several hundred. 
The shock consisted of two distinct parts—the first part being much 
weaker than the second, except at places within an oval area lying 
some miles to the east of the Swansea epicentre. The existence of 
a secondary focus beneath this area is also indicated by the relative 
positions of the isoseismal lines, the isoseismal 8 being much nearer 
the isoseismal 7 at the western than at the eastern end. In twin 
earthquakes, it is difficult to ascertain the position of the focus in 
which the weaker impulse originated, but in the Swansea earthquake 
observations in mines offer an unexpected help. The shock was felt 
severely in mines within an area 8 or 9 miles in diameter, and as 
a tremor outside. The centre of this area les about 1 mile west of 
Llwynypia, 223 miles east of the Swansea epicentre, and is close to 
the centre of the area over which the weaker part of the shock was felt. 
Observations, 53 in number, were obtained from 39 pits, distributed 
over an area 49 miles in length, from near Kidwelly to near Pontypool. 
The shock was, as usual, less strongly felt in pits than on the surface ; 
and the sound was more uniform and monotonous underground. 
Both shock and sound were observed in pits over about the same area. 
In pits not more than 5 miles from the nearest epicentre, the sound 
