230 



REPORT — 1861. 



particles more or less wedge-shaped and flat, and angular fragments more or 

 less crystalline, deposited together, witii 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 find their further compressiliility 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, anotlier condition 

 comes into play : their aggregation of flat, wedge-shaped particles, when thus 

 pressed edgeways, tends powerfully 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 direction 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 without 

 having lost cohesion, or having its elasticity permanently impaired) and com- 

 presses 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 pro- 

 bably due to the mass of the rock being made up of intermixed particles of 

 several different simple minerals, each having specific diff'erences of hardness, 

 cohesion, and mutual adhesion, and which are, in the order of their resist- 

 ances to pressure, in succession broken down, before the final disruption of the 

 whole mass (weakened by these minute internal dislocations) takes place. 



Thus it would appear that the micaceous plates and aluminous clay- 

 particles interspersed 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 finally the elastic skeleton of silex gives way, and the rock is 

 crushed. It is observable, also, that this successive disintegration does not 

 occur at equal pressures, in the same quality and kind of rock, when com- 

 pressed transverse and parallel to the lamination. It follows from this con- 

 stitution of these (and probably of all) rocks that very diff'erent powers of 

 transmitting wave-impulses must arise when the originating forces vary 

 considerably in amount produced of primary compression. It is almost 

 superfluous also to point out the great difterences in wave-transmissive power 

 in directions transverse and parallel to lamination that these experiments dis- 

 close. They prove to us that, in an earthquake shock of given original power, 

 the vibrations will have the largest amplitude when transmitted in the line of 

 lamination, but may be propagated with the greatest velocity in directions 

 transverse to the same, assuming in both cases the Tock solid and unshattered. 



In Table XII. the general results are deduced, and the mean compressions 

 for each of the rocks calculated, and finally the moduli of elasticity are 

 obtained, in pounds and in feet ; the specific gravities adopted in calculating 

 the latter being those given in the body of the paper, as follows : — 



Table XI. 



