lO 



The Irish Naturalist. 



Proceedings of the Royal Dublin Society for corresponding A^ears. 

 It should also be stated that these notes on the geology of Co. 

 Dublin are based on a course of evening lectures given in the 

 Royal College of Science for Ireland in the present 3'ear. 

 Several wider geological considerations were then introduced, 

 which it is here convenient to omit. 



I. — Thk Cambrian Period. 



The oldest rocks in Co. Dublin are those of Howth and the 

 great tract stretching from near Shankill southward into Co. 

 Wicklow, and including there Bray Head and the Sugarloaves. 

 The}' form a series of shales and quartzites, which underlie the 

 Ordovicians (Lower Silurians), and which, consequently, are of 

 Cambrian age, or even older. In the crumpled and wrinkled 

 shales we can recognise the deposits of an old mudd}' sea; these 

 rocks, so well seen at Bra}-, were once mere soft cla3'S, which 

 have since been consolidated by the pressure of other rocks 

 deposited on them, and have been folded and squeezed during 

 long periods of earth-movement. Not only were they bent and 

 uptilted before the Ordovician sea washed against their flanks, 

 but they have since been subject to all the movements that 

 have affected the floor of the country up to the present day. 

 And these movements, as we shall see, have been considerable 

 and repeated. 



The hard quartzites and "quartz-rocks" associated with the 

 shales, present some difficult features. On the south side of 

 Howth, east of the Needles, pink sandstones can be seen 

 beautifully folded ; and one bed lies regularly upon another, 

 clearly deposited, layer upon layer, in the same sea as that 



Fig. I. 



Weathering of jointed quartzite, north of Ireland's Eye, from Howth. 



which formed the shales. And the pocket-lens, and, still 

 better, the microscope applied to thin sections of the rock, 

 show how even the harder masses are made up of grains of 



