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 by 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 8. side of the railway tunnel above mentioned. This 
is an interesting object, as, with the exception of the dyke on 
the shore at Greystones, 24 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 asynclinal basin, through 
the middle of which the intervening valley of Kilmacanogue has 
been denuded. No fossils have been detected thereabouts except 
at the K. 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, hasa 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-« 
