ON THE TRANSIT-VELOCITY OF EARTHQUAKE WAVES. 235 



usually greatest, upon the whole, in planes parallel to bedding or lamination, 

 the transit-rate of shock is most generally fastest in the line of the beds or 

 lamination, rather than across them. 



Or, at least, this latter condition may interfere with the former to the extent 

 of partial, complete, or more than complete obliteration. 



I am not aware that any experiments have previously been made upon 

 the compressibility, &c., of the slate- and quartz-rocks of Holyhead ; and as 

 these rocks are being employed there upon a vast scale for submarine 

 building works, it may not be out of place to draw a few conclusions of a 

 character useful to the practical engineer from the data that have been ob- 

 tained. Some conclusions may be drawn which are applicable to all classes 

 of laminated rocks in the hands of the engineer. 



It is a very prevalent belief that slate-rock (for example), in the 

 form of the sawed rooting-slate of Anglesea or of Valentia (Ireland), will 

 bear a much greater compressive load when the pressure is in the direction 

 of the laniinge, than in one across them. This the preceding experiments 

 Drove to be wholly a mistake — one that has very probably arisen from some 

 vague notion of an analogy with timber compressed the end-way of the 

 grain. 



It is now certain that Silurian slates and quartz-rock, and probably all 

 sedimentary laminated rocks, whether with cleavage or not, are much weaker 

 to resist a crushing force edgeways to the lamina, than across the same, and 

 that the range of compressibility is much greater, for equal loads, in the 

 former direction. 



The facts now ascertained as to the great relative compressibility of lami- 

 nated rock in the direction of the laminae also points out the reason of the 

 great bearing power to sustain impulsive loads, which the toughest and 

 most cohesive examples of slate-rocks, such as the slates of Caernarvonshire, 

 present ; for there can be no grounds to doubt that the high compressibility 

 of rocks of this structure in the plane of the lamina is also accompanied 

 with a high coefficient of extensibility, although probably confined within 

 much narrower limits as to inceptive injury to perfect continuity. 



My experiments point out, that the Silurian slate of Holyhead (the mean 

 both of the hard and the soft) is crushed by a load across the lamina of 

 about 1250 tons per square foot, and that its molecular arrangement is per- 

 manently injured at a little more than 1000 tons per square foot. 



The quartz-rock (the mean of both hard and soft) is crushed by a load, 

 applied in the same manner, of 1630 tons per square foot, and its molecular 

 arrangement is permanently injured at less than 1000 tons per super foot. 

 The quartz-rock gives the highest measure of ultimate resistance, but it is 

 the less trustworthy material when loaded heavily. 



Neither of these sorts of rock, if loaded so as to be pressed in the direction 

 of the lamina, would sustain more than about 0*7 of the above loads at the 

 crushing-point and at that of permanent injury, respectively. From the 

 extreme inequality found within narrow limits in both rocks as quarried, 

 neither should be trusted for safe load in practice with more than about ^^i^th 

 of the mean load that impairs their molecular arrangement, as ascertained 

 from selected specimens, or (say) not to more than 50 tons per square foot 

 for passive or 25 tons per square foot for impulsive loads. 



The high relative compressibility of laminated rocks in the direction of 

 the lamina might probably be made advantageous use of, where they are 

 employed as a building material, for the construction of revetment or other 

 walls of batteries exposed to the stroke of cannon shot, by building the 

 work (under suitable arrangements to obviate splitting up) with the planes 



