662 ME. MALLET ON THE TEANSIT-VELOCITY OF EAETHQUAKE WAYES. 
innumerable minor planes of separation at all conceivable angles to each other in both 
rocks ; and yet we have highly elastic and dense materials forming the substance of both 
rocks, and their general mass remarkably free from open veins, fissures, or cavities. 
We have also two different rocks, the one transmitting the impulse into the other, 
yet neither so widely differing from the other in molecular and other physical charac- 
ters as to make any great or abrupt effect upon the wave at the junction probable. 
In fact, widely to the first glance as the quartz rock and the schist rock appear to 
differ, there is less real distinction of physical character between them than would be 
supposed: both are composed of the same siliceous sand, in about the same size of 
original grains, variously enveloped, in the one in chlorite and mica, and in the other 
in white or grey clay and mica; both have, in ancient geological epochs, doubtless 
derived their materials by degradation and transport from a common source as respects 
their main constituent, the silex ; both have been submitted to approximately similar 
pressures, and probably like temperatures ; and the agglutinating flux has probably 
been mainly the same for both, viz. the minute proportions of alkalies derived from the 
waters of an ancient ocean. 
The main difference in physical structure, viewed upon the broad scale, between the 
quartz rock and the slate is this (as regards our experiments) : — that the great joints 
and planes of separation on the whole approximate to verticality in the former, while in 
the latter, with the exception of some larger faults and dykes, the planes of separation 
are twisted and involved in all directions, but tend more to approach horizontalitij. 
More interesting conditions could thus scarcely be found for experimental determi- 
nation of the transit-rate of earth-waves, or more desirable for future comparison with 
that of earthquake-Vv'aves themselves; much more instructive, indeed, were the actual 
conditions than if the means of experiment presented by these vast quarry operations 
had been in the most regular, undisturbed, and horizontal stratified rock, like some of 
the mountain limestone of Ireland, or the finest and densest laminated roofing-slates 
of Wales. In such ranges we can predict that the transit-velocity would at least be 
high. In the medium chosen for these experiments it was impossible even to guess 
what it might be found. 
I proceed to describe the instrumental arrangements made for the observation of the 
impulse-wave transmitted from the blasts chosen, and for the determination of the 
transit-time along the range of wave-path. Over the surface of solid rock that had 
been chiselled down to a level tabular surface at (O) Pen-y-Brin, a timber shed was 
erected of sufficient size to place the observer, an assistant, and all the instruments 
proper to that spot, under cover and secured from the wind. The side to the N.W. was 
open, to permit of observation along the line of range, with the means of partially closing 
it in high winds. 
Along the line of the boundary- wall of the railway next Pen-y-Brin, and thence along 
up to the highest and most distant point of the quarry cliffs, a line of telegraph-posts 
was planted ; and upon these, two properly insulated iron wires were hung, in such a 
