134 THE Rev. MAXWELL H. CLosgE, 
The accompanying map, drawn by Mr. R. G. Symes, F.G.8., of 
the Irish Geological Survey shows the boundaries of the surface 
exposures of the various formations ; it is therefore unnecessary 
to describe them here. We shall adhere, as far as possible, to the 
chronological arrangement of our subject. 
CAMBRIAN (correlative with the Longmynd rocks; “Lower 
Cambrian” of Sedgwick). Rocks of this formation constitute 
the whole of the Hill of Howth whose highest point is 563 feet 
(the low-lying north-western part of the peninsula is covered 
with Carboniferous Limestone). They occupy also the northern 
and western parts of Ireland’s Eye, the island on the north © 
side of the Howth Peninsula. In the southern part of our dis- 
trict there is a small exposure of them forming the upper part of 
* Carrickgollogan Hill, or Shankill, 912 feet, between the Scalp 
and the sea. They then emerge on the near side of the town of 
Bray and extend thence along the coast for about 14 miles. They 
then leave the coast but still extend southward as far as the lati- 
tude of the town of Wicklow, that is for a length of sixteen 
miles altogether, with a mean width of about five miles, and they 
appear again in the 8.E. part of county Wexford. In addition to 
Shankill, already mentioned, the principal eminences into which 
they rise near the southern part of our district are Bray Head, 
793 feet, the Little Sugar Loaf, 1,120 feet, the Great Sugar Loaf, 
1,659 feet, and the Downs Hill, 1,232 feet. This broken ridge, or 
line of hills, was called by Sir Roderick Murchison, the backbone 
of Ireland. As neither the bottom nor the top of the formation 
is visible, its thickness in this district cannot be ascertained, 
~ ill of Howth —The Cambrian Rocks which form the hill part 
of the peninsula of Howth are generally greenish-grey, some- 
times green and red, grits and slates, with numerous bands of 
quartz-rock, often of considerable thickness. A fine section of 
the rocks is displayed in the sea cliffs along the eastward and 
southern sides of the peninsula, for a leneth of at least three-and- 
a-half miles. The beds are much contorted and faulted; but it 
would appear that they have, in the mass, an E, and W. strike, 
with a general steep dip to the §. | 
Many of the quartzose rocks on the S. side, as also those of 
the N.E. angle of the peninsula, have a peculiar nodular struc-. 
ture, A great number of trap dykes are to be seen in the cliff 
