206 REPORT — 1861. 



boundirig-lines pass off rapidly, hut ffradatim, into the prevailing substance of 

 the rock. It is by no means of equal hardness ; some portions (and these 

 occur without any order or traceable relationship throughout the mass) are 

 much thinner in the foliation, and the layers of chlorite and mica nearly as 

 thick as those of the intervening quai'tz, both being so attenuated, that to 

 the naked eye the edge of the foliation presents only a fine streaky appearance 

 of lighter 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 quarried by the aid of gunpowder, and with very closely formed 

 jumper-holes. 



The specific gravity of the densest portions of the schist rock reaches 2'7G5 ; 

 that of the softer averages 2'7i6. When the rock, whether hard or soft, is 

 broken so that the applied surfaces 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 fractures 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 unprocurable. 

 Both rocks are absolutely dry, or free from all perceptible percolations of 

 surface-water issuing as springs, nor does the rain penetrate their substance 

 by absorption for any appreciable depth, — both indications of their generally 

 compact structure. 



The faults with which our range is intersected, in four places, at a hori- 

 zontal angle of about 75°, are not far from vertical, dipping a few degrees to 

 the N.W. They occur at the points marked f,ff, k, I, on the Geological Sec- 

 tion (Plate III. section 2) ; and the disturbed and shattered plate of rock 

 between each pair I'espectively appears to have sustained a downthrow (or 

 the rocks at either side the contrary) of a few feet, 10 to 12 probably. 

 The surfaces of the walls of these 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 determinations of earth-wave 

 transit are to become useful elements of comparison in the hands of the seis- 

 mologists of other countries with the observed transit-times of natural earth- 

 quake-waves, and a means of controlling such observations, it is essential 

 that the means be afforded of accurately comparing the rock-formation tra- 

 versed in both 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 dispersion, delay, and rapid extinction of the wave of impulse, 

 so far as its structure is concerned, 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 con- 

 volutions, 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 innume- 

 rable 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. 



