674 ME. MALLET ON THE TEANSIT-VELOCITT OF EAETHQHAXE WAVES. 
This great difference of velocity, due to the difference in the molecular properties of 
the material of the rocks in their opposite directions, is, as our Holyhead experiments 
prove, almost wholly obliterated by the vastly increased degree of discontinuity and 
shattering, in the directions approaching that of lamination, or transverse to the wave- 
path in the first case. 
It is necessary to guard against any misconception as to the import of this result. 
The fact ascertained and just enunciated is this, that the velocity of wave-transmission 
is greater in the material of these rocks in a direction across their lamination than in 
one longitudinal to the same, provided or assuming the material he perfectly unshattered 
in both — as homogeneous, in fact, as the small specimen-cubes experimented upon. 
And were the whole mass of the rock, as it lies in its mountain-bed, as homogeneous 
as such cubes, then the velocity of wave-transmission would actually be greater across 
long ranges of natural lamination, than edgeways to them. The opposite, however, is 
often the case ; the wave-transit period is slower as the range of rocky mass is more 
shattered, discontinuous, and dislocated. 
These conditions affect rocks in nature most in or about their planes of bedding, 
lamination, &c., and hence most retard wave-impulses transverse to these planes ; so 
that the more rapid wave-transmissive power of the material of the rock in a direction 
transverse to the lamination may he more than counterhalanced hy the discontinuity of its 
mass transverse to the same direction. 
The results of Wertheim, on the transmission of sound in timber, proved the velocity 
to be greatest in a direction longitudinal to the fibres and annual rings of wood ; less in 
a direction perpendicular to the same, and from the centre of the tree radially towards 
its exterior; and least of all in a direction, guam prox.^ parallel to the annual rings, and 
perpendicular to the longitudinal fibres ; that is to say, that in each case the velocity of 
sound was rapid in proportion to the less compressibility of the wood in the same direc- 
tion. His results might seem at first to conflict with those which I have announced. 
Any such conclusion, however, would be a mistake ; on the contrary, my results per- 
fectly analogize witli those above alluded to. The difference between the cases is, that 
wood in mass, however large, is practically homogeneous and unshattered, and that its 
direction of least compressihility is longitudinal to its laminae (or annual rings) ; whereas 
the direction of least compressihility of rock is transverse to its laminae (which have been 
already powerfully compressed in this direction). In fact, as respects the point here in 
question, there is no true analogy in structure between the lamination (by annual rings) 
of wood, and the lamination or bedding of rock. 
It follows from what precedes, that earthquakes and rocks as both actually occur in 
nature — the rocks being of a stratified or laminated form (generally all sedimentary 
rocks) — must present the following conditions as to rate of transit of shock : — 
1st. If such rocks were perfectly unshattered, and the beds or laminse in absolute 
contact, the shock would be transmitted more rapidly across these than in their own 
direction. 
