]\IE. MAXLET OX THE TEAXSIT-VELOCITY OF EAETHQUAKE WAVES. 659 
The Island of Holyhead, as may be seen on consulting the sheets (Nos. 77 and 78) of 
the Geological Survey of England and Wales, consists mainly of chloritic and micaceous 
schist or slate, and of quartz rock. The latter forms the N.W. portion of the island; 
and in it alone are situated the Harbour quarries, upon the side of Holyhead Mountain 
(as it is called), the same rock rising to its summit, which is 742 feet above the sea, mean 
tide-level. The junction of the quartz and of the schist or slate rock runs in azimuth 
N. 24° E. where it crosses the line of our range, which it intersects at an angle horizon- 
tally of 73° 30'. 
The schist or slate rocks here overlie the quartz, abutting against the flank of the 
latter, apparently unconformably, and having an inclined junction whose dip is towards 
the S.E. and probably, at the place where our range intersects, having an angle of dip 
of about 65° with the vertical. The point of junction is situated about 900 feet from 
the flagstafi’ W ; so that about 2100 feet, on the average of our actual ranges, lay in 
quartz rock, and the remainder, or 3750 feet, in the schist or slate formation, taking 
the mean total range at 5851 feet. The general tendency of the schist is to a dip to the 
N.W., varying from 5° to 20° from the horizontal; but no well-deflned bedding is 
obvious, either in it or in the quartz. 
Lithologically, the quartz rock consists of very variable proportions of pure white, 
light grey, and yellowish quartz, and of white, or yellowish white, aluminous and finely 
micaceous clays. In many places the mass of the rock presents to the lens almost 
nothing but clear and translucent quartz, breaking with a fine waved glassy fracture, 
strildng fire with steel, extremely hard and difficult to break, and showing a very ill- 
defined crj’stallization of the individual particles of quartz, which have all the appear- 
ance of pure quartzose sea-sand that had become agglutinated by heat and pressure 
coacting with some slight admixture of the nature of a flux. 
The specific gravity of such portions, as determined for me by my friend Mr. Robert 
H. Scott, A.M., Secretary to the Geological Society of Dublin, is 2‘656. Erom this 
the rock passes in many places into a softer and more friable material, consisting, when 
minutely examined, of the same sort of quartz-grains, with a white pulverulent clay 
containing microscopic plates of mica disseminated between them. This fractures readily, 
but will still strike fire with steel; and its average specific gravity is 2'G50. 
Both, but particularly the harder variety, are found often in very thick masses of 
nearly uniform quality, separated by great master-joints, though 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 green- 
stone 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 
