TOPOGRAPHY AND GEOLOGY. 3 



the coal-measures of Kilkenny, may be left standing out as a highland, while 

 an anticlinal, fractured at the top and exposed to rapid denudation, may be 

 the first mass to be worn away. The general trend of mountain ranges, 

 however, is determined by the directions of the axes of their folds. 



Before the existence of the Cambrian fauna, which is the first well-marked 

 assemblage of life-forms upon the globe, the still older crust had become 

 locally crushed and folded, giving rise to the " Huronian " system of moun- 

 tain chains. The sediments laid down in periods earlier than the Cambrian 

 were thus converted into gleaming mica-schists and hard flinty quartzites ; 

 limestones became altered into crystalline marbles, and volcanic rocks into 

 tough and dark amphibolites. Molten masses oozed into these from below, 

 baking and often dissolving them, and giving rise, when consolidation took 

 place to granites, and, more especially, to the striped and streaky type of 

 granite known as gneiss. These materials formed the hills and shores 

 against which the Cambrian strata were laid down. In Ireland, there are 

 but few traces of these " Huronian " chains. Yet they existed, and probably 

 underlie part of the north-western highlands. Their gnarled and twisted 

 rocks are clearly visible in Western Sutherland and the Outer Hebrides, 

 and this axis, if continued southward, should reappear in Donegal and 

 Mayo. But, as we shall see later, the existing features of these areas owe 

 most of their characters to the later " Caledonian " folding. 



Blocks of crumpled and gneissic rocks, however, are found included in 



Tyrone in the granites that are connected with the " Caledonian " folds. 



Clearly, then, an ancient gneissic floor existed where Ireland now is, and 



became broken up and involved in all the later movements. A great part 



of the tumbled uplands of the county of Londonderry, 



North-West from Limavady westward, and almost the v/hole of 



Highlands. Donegal, are composed of crystalline rocks which are 



the oldest in the country. Mayo and Connemara also 



continue the same series, until it is lost to sight under Galway Bay. These 



romantic highlands, now carved out into peaks and ridges, with little lakes 



nestling in their hollows, carry us back to a time when Ireland, as we know 



it, had no separate existence, and formed a region on the edge of a great 



continent stretching north towards the pole. 



We do not know if any " Cambrian " rocks were laid down in the Irish 

 area, or if it remained in that period above the sea. Possibly the slates and 

 quartzites of Howth and Bray, and their southern repre- 

 Bray Head sentatives in the lower land near Wexford, belong to 

 Area. the same period as the Cambriati slates of Wales. 



The Great Sugarloaf, in Co. Wicklow, owes its beau- 

 tiful form to the uptilting of a bed of altered sandstone (quartzite) belong- 

 ing to this early series ; the hard rock forms the peak,' and its debris are 

 showered, like a crown of snow, upon the slopes. The broken surface of 

 Bray Head and of the promontory of Howth is due to the resistance of 

 masses of similar quartzite among the more easily weathered slates and 

 shales. 



The " Ordovician " or " Lower Silurian " strata were deposited almost 

 continuously over the Irish area, followed by the Gotlandian {Upper 

 Silurian of the Geological Survey maps). The edge of the northern conti- 

 nent must have dipped beneath the sea, and sands and muds were washed 

 down from it, while beds of limestone, due to the growth of shell-fish and 

 corals, accumulated off its shores. Such limestones are traceable in the 

 Chair of Kildare, and at Portrane, near Dublin, full of Ordovician fossils 



