204 REPORT — 1861. 



constant part of the range between the observing station and the flagstaff, its 

 other permanent terminal, as equal to 4585 (eet, neglecting fractions. 



The base of the staff at S was found to be 68'78 feet above the level of 

 the horizontal surface of the rock at Pen-y-Brin (the observing station O), 

 and the base of the flagstaff at W is 5*70 feet above the same point O. 

 The levelled surface of rock at O is 84 feet above the mean tide-level of the 

 sea in the Asylum Harbour ; and the average rise and fall of spring tides at 

 Holyhead is 18 feet; the line of rock, therefore, through which the range 

 passes is, except as respects surface water, permanently dry to a considerable 

 depth. The majority of the headings are driven into the face of the quarry 

 cliff horizontally, at from 10 to 20 feet above the level of the floor of the 

 quarry, which is on nearly the same level as the point W. Hence, prac- 

 tically, the actual range of transmission through the solid rock of the impulse 

 from each heading when fired, to the seismoscope at the observer's station, 

 may be considered as a horizontal line, and no correction of distance is 

 required for difference of elevation at the two extremities of the observing- 

 range in the reduction of our results. 



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 north-west 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 horizontally 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 south-east, 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 flagstaff W ; so that about 2100 

 feet, on the average, of our actual ranges lay in quartz rock, and the re- 

 mainder, 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 north- 

 west, varying from 5° to 20° from the horizontal ; but no well-defined 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 finally 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, striking fire with steel, extremely hard 

 and difficult to break, and showing a very ill-defined crystallization of the 

 individual particles of quartz, which have all the appearance 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*658. From 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 pulveru- 

 lent clay, containing microscopic plates of mica disseminated between them ; 

 this fractures readily, but will still strike fire with steel, and its average spe- 

 cific gravity is 2'650. 



Both, but particularly the harder variety, are found often in very thick 

 masses of nearly uniform quality, separated by great master-joints, though 



