EARTHQUAKES. 



65 



as we do what the chemical nature of the va- 

 pour of such high tension may be. Encamped 

 on the edge of two craters, on Vesuvius, and 

 on the castellated rock which overlooks the 

 vast gorge of Pichincha, near Quito, I expe- 

 rienced periodical and very regular shocks, 

 and, each time, from 20 to 30 seconds before 

 red-hot ashes or vapours were ejected. The 

 shocks were by so much the stronger as the 

 explosions were later of occurring, and the va- 

 pour consequently had been longer accumula- 

 ting. In this simple fact, confirmed by the ex- 

 perience of so many travellers, lies the general 

 solution of the phenomenon. Active volcanoes 

 are to be regarded as safety-valves for sur- 

 rounding districts. The danger of the earth- 

 quake increases when the opening of the vol- 

 cano is stopped up, and there is no longer a 

 free communication with the atmosphere ; but 

 the destruction of Lisbon, of Caraccas, Lima, 

 Cashmir (1554)^", and of so many towns of 

 Calabria, Syria, and Asia Minor, teaches us, 

 that on the whole the force of earthquakes is 

 by no means greatest in the vicinity of still ac- 

 tive volcanoes. 



As the pent-up force of a volcano acts in 

 shaking the ground, so does the concussion re- 

 act, in its turn, upon the volcanic phenomenon. 

 The occurrence of fissures favours the rise of 

 the cones through which eruptions take place, 

 and the processes which go on within these 

 cones in free contact with the atmosphere. A 

 column of smoke, which had been seen for 

 months rising from the volcano of Pasto, in 

 South America, disappeared suddenly on the 

 occurrence of the great earthquake of Riobam- 

 ba, in the province of Quito, art the distance of 

 48 geographical miles to the south (Feb. 4, 

 1797). After the earth had long continued to 

 tremble in the whole of Syria, in the Cyclades, 

 and in Cuhcea, the convulsions ceased suddenly 

 upon the eruption of a stream, of " red-hot mud" 

 (lava from a crack) in the Lelantine plain, near 

 Chalcis('"). The admirable geographer of 

 Amasia, who has preserved the record of this 

 fact, adds : " Since the mouths of Etna have 

 been opened, through which the fire belches 

 forth, and since, in this way, heated masses 

 and water can be ejected, the lands by the sea- 

 shore are no longer so frequently shaken as 

 they were in times before the separation of 

 Sicily from Lower Italy, when there was no 

 communication with the surface." 



In earthquakes, therefore, we have evidence 

 of a volcano-producing force ; but such a force, 

 as universally diffused as the internal heat of 

 the globe, and proclaiming itself everywhere, 

 rarely gets the length of actual eruptive phe- 

 nomena ; and when it does so, it is only in 

 isolated and particular places. The formation 

 of extensive veins or dykes, in other words, the 

 filling up of fissures with crystalline matter 

 ejected from the interior, such as basalt, mela- 

 phyre, and greenstone, interferes by degrees 

 with the free escape of vapours ; which, con- 

 fined, become operative, through their tension, 

 in three ways : concussively ; explosively, or sud- 

 denly up and down ; and, as first observed in a 

 large portion of Sweden, liftingly or continu- 

 ously, and only in long periods of time per- 

 ceptibly altering the relative level of the sea 

 and land. 

 I 



Before we quit this great phenomenon, which 

 has been here considered not so much in its in- 

 dividual as in its general physical and geognos- 

 tical relations, we must advert to the cause of 

 the indescribable, deep, and quite peculiar im- 

 pression which the first earthquake we experi- 

 ence makes upon us, even when it is accom- 

 panied by no subterranean noises. The impres- 

 sion here is not, I believe, the consequence of 

 any recollection of destructive catastrophes 

 presented to our imagination by narratives of 

 historical events : what seizes upon us so won- 

 derfully is the disabuse of that innate faith in 

 the fixity of the solid and sure-set foundations 

 of the earth. From early childhood we are 

 habituated to the contrast between the mobile 

 element, water, and the immobility of the soil 

 on which we stand. All the evidences of our 

 senses have confirmed this belief But when 

 suddenly the ground begins to rock beneath us, 

 the feeling of an unknown mysterious power in 

 nature coming into action, and shaking the solid 

 globe, arises in the mind. The illusion of the 

 whole of our earlier life is annihilated in an in- 

 stant. We are undeceived as to the repose of 

 nature, we feel ourselves transported to the 

 realm, and made subject to the empire, of de- 

 structive unknown powers. Every sound — the 

 slightest rustle in the air — sets attention on the 

 stretch. We no longer trust the earth upon 

 which we stand. The unusual in the phenom- 

 enon throws the same anxious unrest and 

 alarm over the lower animals. Swine and dogs 

 are particularly affected by it ; and the very 

 crocodiles of the Orinoco, otherwise as dumb 

 as our little lizards, leave the shaken bed of the 

 stream and run bellowing into the woods. 



To man the earthquake presents itself as an 

 all-pervading unlimited something. We can re- 

 move from an active crater ; from the stream 

 of lava that is pouring down upon our dwelling 

 we can escape ; with the earthquake we feel 

 that whithersoever we fly we are still over the 

 hearth of destruction. Such a mental condition, 

 though evoked in our very innermost nature, is 

 not, however, of long duration. When a series 

 of slighter shocks occur in a district one after 

 another, every trace of alarm soon vanishes 

 among the inhabitants. On the rainless coasts 

 of Peru nothing is known of hail, nor of explo- 

 sions of lightning and rolling thunder in the 

 bosom of the atmosphere. The subterraneous 

 noise that accompanies the earthquake there 

 comes in lieu of the thunder of the clouds. Use 

 and wont for a series of years, and the very 

 prevalent opinion that dangerous earthquakes 

 are only to be apprehended two or three times 

 in the course of a century, lead the inhabitants 

 of Lima scarcely to think more of a slight shock 

 of an earthquake than is thought of a hail-storm 

 in the temperate zone. 



Having now taken a general survey of the 

 activity, and likewise of the internal life of the 

 globe : in its contained heat, in its electro-mag- 

 netic tension, in its luminous emanations at the 

 poles, in its irregularly-recurring phenomenon 

 of motion, we come to elementary material 

 PRODUCTION, to chemical changes in the crust 

 of the earth, and in the composition of the at- 

 mosphere, which are in like manner the conse- 

 quence of planetary vital activity. From the 



