Dublin Geology
The county presents several interesting features in its geological relations. Its southern part from Blackrock, Kingstown, and Dalkey forms the northern extremity of the great granitic range which extends through Wicklow and part of Carlow. The granite tract is bordered by a range of incumbent mica slate, which extends eastwards from Shankill and the Scalp to the hills of Killiney, and on the western side commences near Rathfarnham, passes to the south of Montpelier hill, and occupies the upper part of the hollow which separates Seefinane mountain, on the east, from Seechon on the west: in this hollow are displayed some curious intermixtures of the strata of mica slate, granite, and quartz. In the descent from Seechon mountain, both south-westward and north-westward, towards Rathcool, the mica slate passes into clay slate, containing frequent beds of greenstone, greenstone slate, and greenstone porphyry, and occasionally likewise of quartz.
The Tallaght hills consist of clay slate, greenstone, and greenstone porphyry, interstratified; the latter rocks more particularly abounding in the eastern quarter. Rathcoole hills,and the range extending from them south-westward, are composed of clay slate, clay slate conglomerate, and grauwacke slate, alternating with each other. The low group west of Rathcoole is composed of clay slate, grauwacke, grauwacke slate, and granite, of which the last is found remarkably disposed in subordinate beds in the prevailing grauwacke slate of Windmill hill, whence some of them may be traced westward to near Rusty hill. This county contains the only strata of transition rocks known to exist in the eastern part of Ireland. They appear in detached portions along the coast from Portrane Head, by Loughshinny, Skerries, and Balbriggan to the Delvan stream, the northern limit of the county.
The rest of the county, comprising nearly the whole of its plain surface, is based on floetz limestone, commonly of a blueish grey colour, often tinged with black, which colour in some places entirely prevails, especially where the limestone is interstratified with slate clay, calp, or swinestone, or where it abounds in lydian stone. The black limestone in the latter case is a hard compact rock, often of a silicious nature, requiring much fuel for its conversion into lime. Calp, or "black quarry stone," which is generally of a blackish grey colour and dull fracture, and may be considered as an intimate mixture of limestone and slate clay, forms the common building stone of Dublin; it is quarried to a great extent at Crumlin and Rathgar. Besides carbonate of lime, it includes considerable quantities of silex and alumen, traces of the oxydes of iron and manganese, and a small proportion of carbon, which gives to it its dark colour: by exposure to the air it undergoes a gradual decomposition.
The elevated peninsula of Howth consists of irregular alternations of clay slate and quartz rock, both pure and intermixed, on its southern coast the strata present some extraordinary contortions. The only metallic ore at present found in considerable quantity is lead, once abundantly raised near the commons of Kilmainham, and at Killiney; a much more productive vein on Shankill is now being worked by the Mining Company of Ireland. White lead is found in small quantities; the ore is smelted and refined at Ballycorus, in the immediate vicinity of the mine: on Shankill is a tower for the manufacture of shot. At Loughshinny is a copper mine, and at Clontarf a lead mine, both now abandoned. On the south-western side of Howth, grey ore of manganese and brown iron-stone have been obtained in considerable quantities; and a variety of earthy black cobalt ore has been found there.
Coal is supposed to exist near the northern side of the county, and unsuccessful trials have been made for it near Lucan. Among the smaller minerals may be enumerated schorl or tourmaline and garnet, frequently found in the granite; beryl, a variety of emerald, which occurs in several places; and spodumene, which is in great request from its containing eight per cent. of a newly discovered alkali, called lithia, is procured at Killiney, as is also a mineral closely resembling spodumene, designated killinite by Dr. Taylor, its discoverer, from its locality. The limestone strata usually abound with petrifactions, specimens of which, remarkable for their perfection and variety, may be obtained at St. Doulough's, and at Feltrim, about seven miles north-east of Dublin. The shores of the county, particularly from Loughlinstown to Bray, abound with pebbles of all colours, often beautifully variegated, which bear a polish, and are applied to a variety of ornamental uses.