134 The Rev. Maxwell H. Close, 



The accompanying map, drawn by Mr. R. G. Symes, F.G.S., 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. 



Cambeian (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 1 4 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 S.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. 



Hill 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 length 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 S. 



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 



