136 The Rev. Maxwell H. Close, 



quartz-rock strike across the Head. They give rise to the ridges 

 and knobs on the top of the hill. Some of them are cut off by a 

 fault which runs parallel to the shore line, and thus do not 

 appear in the sea-cliff section. One, which is seen on the shore 

 between Periwinkle Rocks and Brandy Hole, and is 50 yards 

 thick, passes across the summit 653 feet and then, by a curious 

 accident, is met at the fault b} r another very similar one which 

 extends for three quarters of a mile farther to near Kilruddery 

 House. Another prominent band of quartz-rock appears on the 

 shore at half-a-mile N. of the point of Bray Head. It is pierced 

 by one of the railway tunnels. It (that is its visible part) is 

 clear of the fault just mentioned and runs across the summit, 

 793 feet, the highest point of Bray Head, and thence extends, as 

 it would seem, continuously by Windgate to near Belmont 

 House, altogether a distance of two miles. These bands of 

 quartz-rock are generally conformable with the stratification of 

 the other rocks ; but it is interesting and important to observe 

 that they sometimes assert their independence of the stratification 

 in a way that is not easy to explain. Mr. G. H. Kinahan, in his 

 "Manual of the Geology of Ireland," contends that this is only 

 to be explained by such quartz-rock (which he distinguishes from 

 quartzite) being intrusive. 



A faulted dyke of greenstone is seen in the coast section, just 

 on the S. side of the railway tunnel above mentioned. This 

 is an interesting object, as, with the exception of the dyke on 

 the shore at Greystones, 2J miles farther S., no other igneous 

 dykes have been found in the Cambrian rocks of that neighbour- 

 hood, although they are so numerous at the Hill of Howth. 



The Little and the Great Sugarloaf are composed of rocks 

 which are lithologically similar to those of Bray Head, but with a 

 greater proportion of quartz-rock, of which their summits are 

 formed. They appear to be contained in a synclinal basin, through 

 the middle of which the intervening valley of Kilmacanogue has 

 been denuded. No fossils have been detected thereabouts except 

 at the E. side of the Little Sugarloaf, and the W. side of the other. 



Greystones, which is situated on the coast, a little beyond the 

 southern margin of our map, has a good exposure of the Cambrian 

 rocks on the shore. These dip, for the most part, steeply north- 

 ward, and contain two massive beds of quartz-rock and the in- 



