ON THE TRANSIT-VELOCITY OF EARTHQUAKE WAVES. 201 



Report of the Experiments made at Holyhead {North Wales) to ascer- 

 tain the Transit- Velocity of Waves, analogous to Earthquake Waves, 

 through the local Rock Formations : by command of the Royal Society 

 and of the British Association for the Advancement of Science. By 

 Robert Mallet, C.E., F.R.S. 

 In my " Second Report on the Facts of Earthquake Phenomena," in the 

 Report of the British Association for 1851, the transit- velocities were expe- 

 rimentally determined of waves of impulse produced by the explosion of 

 charges of gunpowder, and these velocities shown to be — 



In wet sand 824-915 feet per second, 



In discontinuous granite .... 130C)'4'25 feet per second, 



In more solid granite 1664"574 feet per second, 



the range of sand employed having been that of Killiney Strand, and of 

 granite that of Dalkey Island, both on the east coast of Ireland. These 

 results produced some surprise on my own part, as well as on that of others, 

 the transit-velocities obtained falling greatly below those which theory might 

 have suggested as possible, based upon the modulus of elasticity of the 

 material constituting the range in either case. 



I suggested as the explanation of the low velocities ascertained, that the 

 media of the ranges (like all the solids constituting the crust of the earth) were 

 not in fact united and homogeneous elastic solids, but an aggregation of solids 

 more or less shattered, heterogeneous, and discontinuous; and that to the loss 

 of vis viva, and of time in the propagation of the wave from surface to sur- 

 face, was due the extremely low velocities observed. 



The correctness of this view, and a general corroboration of the correct- 

 ness of the experimental results themselves, have since been made known by 

 the careful determinations by Nbggerath and Schmidt respectively, of the 

 transit-velocities of actual earthquake waves in the superficial formations of 

 the Rhine country and of Hungary, and by myself in those of Southern Italy, 

 all of which present low velocities coordinating readily with my previous 

 experimental results. 



In the Report above mentioned, I suggested the desirableness of extending 

 the experimental determination of wave-transit to stratified and foliated 

 rocks, as likely to present still lower velocities than those obtained for shat- 

 tered granite, as well as other important or suggestive phenomena. The 

 operations in progress at the Government quarries at Holyhead (Island of 

 Anglesea, North Wales), of dislodging vast masses of rock by means of gun- 

 powder for the formation of the Asylum Harbour there, appeared to me to 

 present a favourable opportunity of making some experiments upon the stra- 

 tified rock formations of that locality, by taking advantage of the powerful 

 explosions necessary at the quarries. These quarries are situated (see Map, 

 PI. II.) on Holyhead Mountain, on its N.E. flank, in metamorphic quartz 

 rock, and in 1852 (a vast mass of material having been already removed) 

 presented a lofty, irregular, and nearly vertical scarp, reaching to 150 feet in 

 height above the floor of the quarry in some places. 



From this wall of solid rock the process of dislodgement was continued, 

 not by the usual method of blasting, by means of small charges fired in 

 jumper-holes bored into the rock, but by the occasional explosion of large 

 mines, containing at times as much as ni)ie tons of gunpowder lodged in one 

 or in three or more separate foci deep within the face of the clifi", and formed 

 by driving " headings" or galleries from the base of the mural face into the 

 rock. From the charges of powder placed in bags at the innermost extre- 

 mities of these headings, which were stopped up by several feet of " tamping '* 



