426 EARTHQUAKE PHENOMENA. 



plumb above the centre of impulse) is subject to great and often very 

 perplexing and abrupt changes in azimuth and direction in very moun- 

 tainous or shattered country, or even in perfect planes of deep allu- 

 vium (like the basin of the Ganges) resting upon a highly uneven 

 skeleton of rock, or where the formations vary suddenly and much, 

 or are very discontinuous. The change often amounts in direction 

 to total inversion, and in azimuth to 90°. 



Great perturbation of direction is also produced by the abutting of 

 one mountain chain upon another, which usually alters the apparent 

 angle of emergence also. The methods of disentangling these larger 

 and complex phenomena exceed the limits here imposed. — (See 4th 

 Report Brit. Ass. Trans., 1857-' 58.) 



Amongst doubtful phenomena on record are inversions of bodies, 

 such as parts of pavements turned upside down, &c. ; such cases, or 

 any strange and unaccounted for phenomena, deserve special atten- 

 tion. — (See 1st Report, '"Facts of Earthquakes," section 6, Secondary 

 Effects; " Cosmos," vol. iv., Sabine's Translation.) 



In traversing an extensive city, or thickly-built-over country, to 

 observe the shattered buildings — having first ascertained generally 

 the line of motion of the wave — the observer should remark where 

 its directou of motion has appeared to change as it passed along, and 

 note all the conditions that seem to have there affected it. He should 

 also obtain decisive evidence of its actual transit, for sometimes the 

 wave seems to emerge all but simultaneously over a vast tract of 

 country, where the origin is deep-seated, and nearly vertically below. 

 Changes in the rate of transit horizontally, or in the energy of the 

 wave, should be noted by its effects on similar objects at distant 

 spots. These changes may be expected at the lines of junction of 

 different rocks or other formations. Evidence should also, if possi- 

 ble, be got of any breaking up of the primary wave into secondary 

 waves, as of several shocks being felt where only one has occured 

 further back. 



All evidence should, as far as possible, be circumstantial. Nature 

 rightly questioned never lies; men are prone to exaggerate, at least 

 where novel and startling events are in question. 



Various local conditions must be recorded: the great features of the 

 geological formations of the region, not only the successive under- 

 lying rocks, with the general directions of bedding, lamination, 

 joints, &c, but the topographical character of surface, the directions 

 and altitudes of the chief mountain ranges and of the main river 

 courses, the depth and description of its loose materials, their varia- 

 tions and extent, and the same for the surrounding districts, from 

 Avhence and towards which the earth-wave travels especially. The 

 deeper a knowledge can be got by exposed sections, &c, of the rocks 

 of the shaken district the better; the proximity or otherwise to vol- 

 canic vents, active or passive, the lithological character of material 

 of the country shaken, whether broken, solid, or fissured; if the hitter, 

 their general directions, dip, &c, whether dry or flooded, and the 

 effects on the transit of the wave, of changes in any or all of these 

 conditions; places least and most affected by the shock, and whether 



