]ME. iVrALLET OX THE TEAXSIT-YELOCITT OF EAETHQUAEE WAVES. 661 
out any order or traceable relationship throughout the mass) are much thinner in the 
fohation, and the layers of chlorite and mica nearly as thick as those of the intervening 
quartz, both being so attenuated, that to the naked eye the edge of the foliation pre- 
sents only a fine streaky appearance of fighter and darker green-grey tint. The softest, 
however, readily strikes fire with steel ; and throughout the whole mass of the rock, for 
the length of our range, it is so hard, coherent, and intractable as to be only capable of 
being quamed by the aid of gunpowder and with very closely-formed jumper holes. 
The specific gra\ity of the densest portions of the schist rock reaches 2'765, that of 
the softer averages 2 '746. When the rock, whether hard or soft, is broken, so that the 
applied siu'faces of the foliations are -visible, they are often found glistening and greasy 
to the feel, from flattened microscopic scales of mica, or possibly of talc. 
The quartz rock fractiues under the effect of gunpowder into great lumpy masses, 
with much small rubbish. The schist under^ that, from jumper-hole blasts, breaks up 
into coarse, angular, knotted and most irregular wedges, the foliations breaking across 
in irregularly receding steps, and (throughout our range at least) a stone with a single 
flat bed being perhaps unprocm-able. Both rocks are absolutely dry, or free from all 
perceptible percolations of surface-water issuing as springs, nor does the rain penetrate 
then* substance by absorption for any appreciable depth, — both indications of their gene- 
rally compact structure. 
The faults with which our range is intersected in four places, at a horizontal angle 
of about 7 5°, are not far from vertical, dipping a few degrees to the N. W. They occur 
at the points marked f, g, Jt\ Z, on the Geological Section (Plate XXI. Section II.) ; and 
the disturbed and shattered plate of rock between each pair respectively appears to 
have sustaineel a downthrow (or the rocks at either side the contrary) of a few feet, 10 
to 12 probably. The surfaces of the w'alls of those faults, so far as I can judge from 
rather imperfect superficial indications, appear to be in close contact ; and such is the 
character of all the small faults that intersect the formation hereabouts. 
I have been thus tediously minute in describing the character of the rocks throughout 
our range because, if experimental determmations of earth-wave transit are to become 
useful elements of comparison, in the hands of the seismologists of other countries, -with 
the observed transit-times of natural earthquake-waves, and a means of controlling such 
observations, it is essential that the means be afibrded of accurately comparing the rock 
formation traversed in all cases. 
From what has been described, it will be remarked that the rock here chosen for 
experiment presents in the highest degree the properties capable of producing disper- 
sion, delay, and rapid extinction of the wave of impulse, so far as its structure is con- 
cerned, although the modulus of elasticity of a very large proportion of its mineral 
constituents (silex) is extremely high, and its specific gravity as great as that of Dalkey 
granite. Added to its minutely foliated and mineralogically heterogeneous character, 
with its multiplied convolutions, we have five great planes of transverse separation in 
the range, one of these forming the plane of junction of the quartz and schist, with 
