As an example of a volcanic outburst terminating a seismic disturbance, may be mentioned the appearance of a new volcano in the centre of Lake Ilopango, as a sequel to the shocks which had disturbed that neighbourhood in 1879.[121] In 1750 there were continuous shakings lasting over three months at Manilla. These terminated with an eruption of a small island in the middle of a neighbouring lake. Three days after the commencement of this eruption, four other small islands rose in the same lake.[122] Antonio d’Ulloa, when speaking of the Andes, remarks that after a volcanic eruption the shocks cease.[123] _Conclusion._—Looking at this question generally, insomuch as the greatest number of volcanic eruptions appear, according to Fuchs, to have taken place in summer, whilst the greatest number of its earthquakes have apparently taken place in winter, it would seem that the two phenomena are without any direct connection, unless it be that both are different effects of a common cause. Regarded in this manner, an earthquake may be looked upon as an uncompleted effort to establish a volcano. To use the words of Mallet, ‘The forces of explosion and impulse are the same in both; they differ only in degree of energy, or on the varying sorts and degrees of resistance opposed to them.’[124] Although we have many examples of earthquakes having occurred without volcanic eruptions, and, on the other hand, of volcanic eruptions without earthquakes, volcanoes may still be regarded as ‘safety-valves of the earth’s crust,’ which, by giving relief to internal stresses, guard us against the effects of earthquakes. That many earthquakes are felt at Copiapo is attributed to the fact that in the neighbouring mountains there are no volcanic vents. We must not, however, overrate the protective influence of volcanoes. In the Sandwich Islands we see the columns of liquid lava in neighbouring mountains standing at different heights, indicating a want of subterranean connection between these vents. In consequence of this it would seem that enormous pressures might be generated in the neighbourhood of one of these mountains without finding relief at the other. When we have conditions like these, it would seem that the eruption of a volcano may have little or no influence in protecting neighbouring districts. This may possibly be the explanation of the fact that in 1835 Concepcion was destroyed, notwithstanding there being an unusual activity in the volcanic vents of the neighbouring mountains. CHAPTER XVII. THE CAUSE OF EARTHQUAKES. Modern views respecting the cause of earthquakes—Earthquakes due to faulting—To explosions of steam—To volcanic evisceration—To chemical degradation—Attractive influence of the heavenly bodies—The effect of oceanic tides—Variation in atmospheric pressure—Fluctuation in temperature—Winds and earthquakes—Rain and earthquakes—Conclusion.