ME. MALLET ON THE TEANSIT-VELOCITY OF EAETHQUAKE WAVES. 675 
The difference is more in favour of the transverse line, in proportion as the rock is 
made up more of angular sedimentary particles of very unequal dimensions, the longest 
being parallel to the general lamination, and in proportion as the imbedding paste is 
softer in relation to such particles. 
Some sedimentary rocks no doubt exist, made up of particles perfectly uniform and 
equal in all three dimensions, and without imbedding paste — such as the lithographic 
stones of Germany, the Apennine marl-beds, &c., in which (assuming the above con- 
dition as to continuity) the transit-period would probably be alike in all directions. 
2nd. The actual amount of shattering and discontinuity in nature being 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 experiments have previously been made at all upon the com- 
pressibility, &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 obtained. 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 
roofing-slate of Anglesea or of Valentia (Ireland), will bear a much greater compressive 
load when the pressure is in the direction of the laminae, than in one across them. This 
the preceding experiments prove 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 compressi- 
bility is much greater, for equal loads, in the former direction. 
The fact now ascertained, as to the great relative compressibility of laminated 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 incipient 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 applied across the lamina of about 1250 tons per 
square foot, and that its molecular arrangement is permanently injured at a little more 
than 1000 tons per square foot. 
4 z 
MDCCCLXII. 
