ii8 



NA TURE 



IDcc. 9, 1880 



shock. In not ;i few instances these seem to have been 

 the most alarming part of the phenomena. They are 

 variously likened to the sound of a rushing wind, the roll of 

 waggons, the muttering of thunder, or the rattle of cannon. 

 With these aerial vibrations there are also recorded sounds 

 as of a sudden snap or blow, or explosion in the earth 

 underneath. Another feature of the earthquake-register 

 is the persistence with which a relation is believed to 

 exist between the commotion in the ground and the 

 state of the atmosphere above. In some cases, indeed, 

 the barometer is said to have suddenly fallen, and 

 then to have risen after the shock had passed. Warm, 

 damp, moist weather, heavy rain, thunder, strange 

 electrical discharges, fire-balls, and other meteoric phe- 

 nomena are chronicled as the concomitants of earthquakes. 

 It may be said, of course, that the occurrence of these 

 events together is only of the nature of a coincidence, 

 and cannot conceivably be anything else. There can be 

 no doubt, however, that in Britain, as on the Continent, 

 earthquakes have been more numerous in the winter than 

 in the summer half of the year. Of the fifty-nine earth- 

 quakes in Sir John Prestwich's list, as Prof. Prestwich has 

 pointed out, ele\-en occurred in winter, ele\-en in spring, 

 se\en in summer, and eight in autumn. Out of 139 

 earthquakes recorded as ha\ing happened in Scotland up 

 to September, 1839, eighty-nine occurred in the winter 

 half of the )-ear and fifty in the summer half. We 

 cannot suppose that any variation in the meteorological 

 condition of the atmosphere can directly give rise to an 

 earthquake. Ne\'ertheless it is conceivable that where 

 the crust of the earth is in a condition of tension, rapid 

 and extensive changes of atmospheric pressure may 

 destroy an equilibrium that has pre\-iously been barely 

 naaintained. The obser\-ed relation between a low 

 barometer and the more copious escape of fire-damp 

 within coal-mines may possibly be of wider appli- 

 cation. 



It is evident, moreover, that the source of disturbance 

 must be at no great depth from the surface. This is 

 shown b)- the markedly local character of the pheno- 

 mena. A shock of considerable violence which rends 

 walls, overturns chimney-pots, rings bells, shakes fur- 

 niture, and fills with alarm the inhabitants of a few 

 parishes, but is quite unperceived in the districts around, 

 cannot have a deep-seated origin. In looking at the 

 districts specially liable to such visitations we notice 

 in some degree a connection with geological structure. 

 The earthquake area in the south-west of England em- 

 braces within its borders the ranges of the Malvern and 

 Mendip Hills, which, with the surrounding country, point 

 to a long succession of geological disturbances, while the 

 hot springs that still rise there furnish additional indi- 

 cations of a connection between the heated interior and 

 the surface. The most remarkable earthquake district in 

 these islands at present is undoubtedly that of Comrie in 

 Perthshire; wherein the month of October, 1839, no fewer 

 than sixty-six shocks were felt, the severest being per- 

 ceived as far north as Dingwall, and as far south as Cold- 

 stream. During the last forty years the British Association 

 has appointed two Committees to investigate the nature of 

 the shocks so frequently experienced there. But their 

 labours cannot be said, to have as yet thrown much light 

 on the subject. They have erected seismometers of 



approved construction and sensitiveness, but in many 

 cases shocks that have been distinctly perceptible to the 

 inhabitants have not been registered by the instruments. 

 Much speculation has been offered as to the cause that 

 earth-tremors should be specially abundant in that dis- 

 trict. Reference has been made by difierent observers 

 to protrusions of granite and dykes of basalt which 

 traverse the rocks, as if these igneous masses supplied 

 a clue to the source of movement. But neither the 

 granite bosses nor the dykes are specially conspicuous 

 in the Comrie district. On the contrary, they are 

 there small in area and few in number compared with 

 their occurrence in other tracts where earthquake shocks 

 are rare. A geological structure at Comrie, however, 

 which so far as we are aware has not been dwelt upon in 

 this connection, is the occurrence there of the great 

 fracture by which the southern edge of the Scottish 

 Highlands is bounded. The Old Red Sandstone with its 

 associated volcanic bands has been thrown on end against 

 the crystalline schists. Of the extent of the dislocation no 

 precise measurements ha\-e yet been made ; probably the 

 amount of upthrow varies along the line. At the north- 

 eastern end of the fracture the sandstones and con- 

 glomerates have been placed on their ends for about two 

 miles back from the fault. The line of dislocation can 

 be traced across the island from sea to sea and across the 

 island of Arran, whence it points for Ireland. It is 

 probably one of the largest, as it certainly is the longest, 

 fracture within the British area. On its north-western 

 side lie the crumpled schists of the Highlands ; on its 

 south-eastern boundary are the dislocated, curved, and 

 even inverted strata of the Old Red Sandstone. Two 

 series of rocks of very different structure and elasticity 

 are here brought abruptly together along a \-ertical or at 

 least steeply inclined face, which must descend for several 

 thousand feet from the surface. So far therefore as 

 geological structure can be supposed to govern the origin 

 and eft'ects of earthquakes there does not appear to 

 be within these islands any line or district where ter- 

 restrial disturbances should be so readily felt as along 

 the flanks of the Scottish Highlands. Shocks coming 

 from the Lowlands will recoil against the crystalline wall 

 of the Highland schists, and be consequently more per- 

 ceptible there than over the snore homogeneous formations 

 lying to the south. Another area in which earthquakes 

 have been frequently observed is that of the Great 

 Glen. This longest, straightest, and deepest of British 

 valleys has from early geological times been a line of 

 weakness. 



There seems every probability in the supposition that 

 some at least of our earthquakes result from the sudden 

 collapse of rocks that have been under great strain. 

 Their occurrence along lines of powerful fault suggests 

 tha thet rocks on one or both sides of these dislo- 

 cations are still subject to great tension, and that occa- 

 sional relief is obtained by a snap which is powerful 

 enough to generate an earthquake, though it gives 

 rise to no change of level at the surface. When we 

 reflect upon the constant strain on the terrestrial crust 

 as it settles down upon the more rapidly contract- 

 ing nucleus, we may be allowed to be grateful that 

 earthquakes are not eveiywhere more numerous and 

 destructive. 



