660 ME. m:allet on the teansit-velocitt oe eaethqijake waves. 
of which may be seen as cutting through our line of range at/, in Plate XX. 
Section II. 
At a short distance heliind the quarry cliff, and seat of our several explosions, a great 
clay dyke occurs in the quartz rock — a wall, in fact, of about 20 feet in average thick- 
ness, running in the direction marked on the Map (Plate XX.), and with a dip of only 
about 20° from the vertical. This consists of strongly compacted clay, nearly pm’e 
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 expe- 
rimental 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 
dii'ection 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 chlo- 
rite and mica, present planes of weakness and of separation ; the rock is, in fact, every- 
where 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 com- 
monly 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 4 or 5 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 portions 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 convoluted towards the steeper sides, as though these masses 
had shpped 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 directions, and without any 
relation to the existing surface of the country. Everywhere this mass of minutely 
structured, convoluted, and foliated rock is cut through by joints of separation, mth 
surfaces in direct and close contact, and by thin seams and veins of hard and sometimes 
pretty well crystalhzed 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 bounding-hnes pass off rapidly, but gradatim, into the prevailing substance 
of the rock. It is by no means of equal hardness. Some portions (and these occur with- 
