ME. MALLET ON THE TEANSIT-VELOCITY OF EAETHQUAKE WAVES. 669 
In this direction of compression, the hardest slate is more than double as compressible 
as the hardest quartz. 
When compressed transverse to the lamina, however, the hard slate and hard quartz 
prove to have very nearly the same coefficient of compressibility, which is very small for 
both ; while the softest slate and the softest quartz, compressed in the same way (trans- 
verse to lamina), have also nearly the same coefficient of compressibility, but one about 
four times as great as for the hardest like rocks. 
These facts point towards the circumstance of the original deposit and formation of 
these rocks as their efficient causes. Both rocks consist of particles more or less wedge- 
shaped and flat, angular fragments more or less crystalline, deposited together, with 
their larger dimensions in the planes of lamination, which lamination has been produced 
by enormous compression in a direction transverse to its planes. Hence the mass of 
these rocks has already been subjected to enormous compression in the same direction 
as that in which we now And their further compressibility the least. But, besides that 
we might from this cause alone anticipate a higher compressibility when the pressure is 
applied to them parallel to the lamination, another condition comes into play; their 
aggregation of flat, wedge-shaped particles, when thus pressed edgeways, tends power- 
fully to their mutual lateral expansion, and hence to their giving way in the line of 
pressure. 
The per-saltum way in which all the specimens of both rocks yield, in whatever direc- 
tion pressed, is another noteworthy circumstance. On examining the Tables I. to VIII. 
it will be seen that the compressions do not constantly advance with the pressure, but 
that, on the contrary, the rock occasionally suffers almost no sensible compression for 
several successive increments of pressure, and then gives way all at once (though with- 
out having lost cohesion, or having its elasticity permanently impaired) and compresses 
thence more or less for three or four or more successive increments of pressure, and then 
holds fast again, and so on. This phenomenon is probably due to the mass of the rock 
being made up of intermixed particles of several different simple minerals, having each 
specific differences of hardness, cohesion, and mutual adhesion, and which are, in the 
order of their resistances to pressure, in succession broken down, before the final disrup- 
tion of the whole mass (weakened by these minute internal dislocations) takes place. 
Thus it would appear that the micaceous plates and aluminous clay-particles inter- 
spersed through the mass give way first. The chlorite in the slate, and probably felspar- 
crystals in the quartz-rock, next, and so on in order, until Anally the elastic skeleton of 
silex gives way, and the rock is' crushed. It is observable, also, that this successive dis- 
integration does not occur at equal pressures, in the same quality and kind of rock, 
when compressed transverse and parallel to the lamination. It follows from this con- 
stitution of these (and probably of all) rocks that very different powers of transmitting 
wave-impulses must arise when the originating forces vary considerably in amount of 
primary compression. It is almost superfluous also to point out the great differences in 
wave-transmissive power in directions transverse and parallel to lamination that these 
