162 



KNOWLEDGE. 



[July 1, 1895. 



designated earthquakes, there are the earth tremors and 

 earth movements that can only be noted by the delicate 

 seismograph. Disturbance of the exterior rocks of the 

 globs, at one part or another of the earth's surface, must, 

 therefore, be very frequently, it may be safe to say daily, 

 taking place. 



The conclusion, consequently, appears irresistible that 

 the cause of earthquakes cannot, with a due regard to 

 absolutely incontrovertible geological facts, be attributed to 

 a contraction or shrinkage of the bulk of the globe, and 

 that, therefore, another cause must be found. 



Any cause, to be adequate for the production of con- 

 stantly recurring phenomena, must be constantly operating 

 and the result of forces continuously acting. So far as 

 our present knowledge extends, there are but two classes 

 of forces capable of disturbing the surface rocks of the 

 globe. These are (1) physical and (2) chemical. By 

 expansion and contraction consequent upon alteration of 

 temperature, lateral pressure and lateral tension of incal- 

 culable intensity and power may be produced. By chemical 

 action the requisite alteration of temperature to cause 

 alteration of density, and consequently alteration of bulk, 

 may be produced, to say nothing of the evolution of gases 

 by decompositions and reactions. Again, chemical action 

 may be checked and prevented or suppressed by excessive 

 pressure, and stimulated or permitted by a diminution of 

 pressure, and as lateral pressure lessens vertical pressure, 

 increase of heat from slight chemical action, occasioning 

 expansion and therefore lateral pressure, may be the cause 

 of relief of vertical pressure, with the result of allowing 

 more intense chemical action productive of greater heat 

 and still greater expansion, with proportionally increased 

 lateral pressure. 



From these considerations it is obvious that physical 

 and chemical forces act and react on each other, and in 

 combination are capable of producing surface phenomena 

 of great magnitude and importance, as well as of a minor 

 character. Here, then, are forces constantly acting or 

 potentially existing that are quite adequate to the pro- 

 duction of seismic phenomena, without postulating the 

 shrinkage of a thin crust over a fused interior mass that 

 is alike opposed to the observations of astronomers, the 

 calculations of physicists, and the facts of geology. It is 

 true that the hypothesis of a solid nucleus with an inter- 

 mediate ocean of fused matter between it and the solid 

 exterior surface crust, as the source of lava, has recently 

 received the support of eminent physicists ; but this 

 requires a mere thread of lava, dependent for its fluidity 

 ' on a temperature rapidly lost, finding its way as a fluid 

 through a thickness of thirty miles of solid, and therefore 

 comparatively cool rocks, which certainly appears to be 

 quite impossible. 



In the early part of the century Sir Humphry Davy, 

 after his discovery of the elements potassium and sodium 

 and their violent combination with the oxygen of water, 

 advanced a chemical theory to explain volcanic action, 

 and, later. Dr. Daubeny also favoured a chemical hypothesis. 

 These views have, however, been generally discarded as 

 inadequate, but chemical forces and physical forces acting 

 in conjunction appear to be amply suliicient to cause, not 

 only seismic, but volcanic, action also. 



In the year 1888, 1 brought before the British Association 

 an hypothesis that seemed to me to account satisfactorily 

 for volcanic action, and to meet the requirements of its 

 observed phenomena.* By the hypothesis then explained 



* " On the Causes of Volcanic Action," Report of the British Asso- 

 ciation for 1888 (Bath meeting), p. 670; "Proceedings of the Geolo- 

 gists' Association," vol. xi., p. i ; " Mount Vesuvius," p. 212. 



and formulated, subterranean igneous conditions were 

 attributed to chemical action when allowed by favouring 

 physical conditions, prominent amongst which was dimi- 

 nution of pressure. To the same physio-chemical agency 

 I attribute earthquakes, and earthquake shocks and 

 tremors. 



Earthquakes and earthquake shocks are not infrequent in 

 the neighbourhood of active volcanoes, and minor tremors 

 are common on volcanoes during and preceding eruptions. 

 All such seismic phenomena are doubtless due to volcanic 

 action, and, therefore, are primarily caused by what has 

 produced that action. But the earthquakes of non-volcanic 

 regions, which have their centres far away from any active 

 vent, require a further explanation. They are caused, it 

 appears to me, by the same chemical action that originates 

 volcanic phenomena, but, acting with less intensity, it does 

 not bring about rock-fusion, on which volcanic action 

 depends. It is sufficient, however, to produce heat, gases 

 and vapours with accompanying local expansions and 

 succeeding contractions, and thus it occasions deep-seated 

 and sudden fractures that give rise, from separate and 

 distinct dynamic foci, to earth vibrations, which at the 

 surface cause earthquakes, and earthquake shocks and 

 tremors. 



According to these views, seismic action and those 

 of volcanic and plutonic origin have this in common, 

 that they each originate from chemical action arising and 

 developing from favouring physical conditions. When 

 that action is sufficiently intense to create a rock-fusing 

 heat, then either volcanic or plutonic results will follow ; 

 and when the heat produced at any one focus of chemical 

 change, though considerable, is insufficient to fuse the 

 adjacent rocks, an earthquake may be caused. 



The sources of seismic, volcanic, and plutonic action 

 will, therefore, be in a thin outer rind of the globe resting 

 on an interior solid foundation, and unconnected with any 

 fused central mass, and, consequently, with the exception 

 of regions of fused rock near the exterior, the earth as a 

 whole may be solid to the centre, and this would be quite 

 in accordance with the rigidity our planet has been proved 

 to possess. 



THE EFFECTS OF LIGHTNING ON TREES. 



THE efi'ect of lightning on the oak and the beech is 

 very diff'erent. As a rule, the effect of a lightning 

 stroke on the oak tree is to split the trunk, or sever 

 the large square-set branches. This is well 

 exemplified in the accompanying illustration, 

 reproduced from an excellent photograph kindly lent by 

 Mr. H. .J. Adams, of Beckenham. 



The tree — originally a fine specimen, some fifty or sixty 

 feet high, and absolutely sound — stood fairly isolated in a 

 copse near Cobham in Kent. The whole of the crown was 

 shattered, only about fifteen feet of the trunk and two of 

 the lowermost branches being left. The bark, as the 

 illustration shows, was entirely stripped from the trunk, 

 which, though split down to the root, showed practically 

 no signs of scorching. 



The eff'ect of lightning on a beech tree is not so apparent 

 at the time, or even for long after. This is partly because 

 the sappy branches of beeches are good water conductors, 

 the water generally descending one side of the trunk, at 

 least to a great extent, and the result is that the lightning 

 follows the water, and scorches the bark down to the earth. 

 The damage is frequently not visible on the surface of 

 the bark till the spring following. In a year or two, 

 however, the tree dies and becomes a picturesque ruin. 



