ON THE TRANSIT-VELOCITY OP EARTHQUAKE WAVES. 205 



scarcely to be considered as beds ; but usually the mass, viewed in the large, 

 is heterogeneous in the highest degree, massive and thick in one place, full 

 of joints and even minutely foliated in others, and everywhere intersected by 

 thin and thick veins of harder quartz, agglutinated sand, and, elsewhere, 

 friable sand, and of soft sandy clay. 



Both the quartz rock and the schist of the island are intersected by three 

 great greenstone dykes (of inconsiderable thickness, however), none of them 

 interfering with our range, and by one or more great faults, all of which 

 run through nearly the whole island in a N.W. and S.E. direction, and by 

 numerous other minor faults and dislocations, some of which may be seen 

 as cutting through our line of range ^A f, g, k, I, in Plate III. section 2. 

 No. 11. 



At a short distance behind the quarry cliff, and seat of our several ex- 

 plosions, a great clay dyke occurs in the quartz rock — a wall, in fact, of. 

 about 20 feet in average thickness, running in the direction marked on the 

 Map (Plate II.), and with a dip of only about 20" from the vertical. 

 This consists of strongly compacted clay, nearly pure white, and more or 

 less mixed with fine sand and grains of mica, but cannot be called rock, 

 though continually passing into stony masses. Lying as it does in rear of our 

 experimental headings, it was of some value, as presenting a dead solid anvil 

 to the pulse from each explosion, in the contrary direction to that of the 

 observed wave of impulse, and hence causing a larger and more distinctly 

 appreciable wave to be transmitted in the direction towards the seismoscope. 



The schist rock, in colour, passes from fawn-colour and light-greenish 

 ashen-grey into a rather dark tea-green. It owes its colour to disseminated 

 thin layers of chlorite, and probably of black or green mica in minute scales, 

 between which are thicker layers of quartz, presenting identically the same 

 mineral characters as those of the quartz rock beneath. These layers, owing 

 to the small relative hardness and cohesion of the chlorite and mica, present 

 planes of weakness and of separation ; the rock is, in fact, everywhere thinly 

 foliated, the average thickness of a plate seldom exceeding 0*2 of an inch, and 

 averaging about one-half that thickness. These foliations are twisted, bent, 

 doubled up, and distorted in every conceivable way : the contortions are 

 often large, the curves having radii of some feet, with minor distortions 

 within and upon them ; but most commonly they are small ; so that it is rare 

 to get even a hand specimen presenting flat and undistorted foliations, while, 

 quite commonly, hand specimens may be found presenting, within a cube of 

 four or five inches, two or three curves of contrary flexure, often in all three 

 axes, and with curvatures short, sharp, and abrupt, almost angular. There 

 is a general tendency observable in the greater convolutions to conform more 

 or less to the surface contour of the country ; so that the largest and flattest 

 folds are found to occupy, with an approach to horizontality, the topmost por- 

 tions of the great humps or umbos of schist rock that form the characteristic 

 of the landscape, and so rolling off in folds smaller, steeper, and more con- 

 voluted towards the steeper sides, as though these masses had slipped and 

 doubled upon themselves when soft and pasty. 



Occasionally, however, where deep cuttings have exposed the interior of 

 such surface-knolls, it is found sharply convoluted and twisted in all direc- 

 tions, and without any relation to the existing surface of the country. Every- 

 where this mass of minutely structured, convoluted, and foliated rock is cut 

 through by joints of separation, with surfaces in direct and close contact, and 

 by thin seams and veins of hard and sometimes pretty well crystallized quartz, 

 now and then discoloured by oxide of iron, and with minute cavities filled 

 with chlorite and mica, and with others of agglutinated quartzose sand, whose 



