BOTANICAL FEATUEES OF IRELAND xxiii 



tinuity is first broken by the Old Red Sandstone ridge of Slieve 

 Bloom and Devil's Bit, an anticline that runs SW. from Mount- 

 mellick in Queen's County down into the North Tipperary highlands. 

 But for botanical purposes the Central Plain may be considered as 

 lapping round these hills, and stretching southward to the Suir, 

 where it abuts on the great mountain-wall of the Galtees and 

 Comeraghs. The average elevation of the plain is not more than 

 two hundred feet above sea-level. The Grand Canal, which crosses 

 from the east coast at Dublin, to the Shannon, attains its summit 

 level at 279 feet in Kildare. The Royal Canal, with a rather more 

 northerly course from Dublin, rises to 324 feet near MuUingar. 

 Both of these summits are on the main watershed, which runs 

 north and south about half way between the Irish Sea and the 

 Shannon. The surface of the Central Plain is seldom absolutely 

 flat. Low cultivated ridges and occasional knobby hills alternate 

 with stretches of pasture and marshy meadow-land, and with the 

 great red bogs. The absence of dominating ridges causes the river- 

 basins to be basins in name only, and often indistinguishable in the 

 field. This it is that makes any scheme for the subdivision of 

 Ireland by river-basins a geographical, but in no sense a phyto- 

 logical one. The same feature is accountable for the singularly 

 extensive views that meet the eye of the traveller who ascends one 

 of the few eminences of the Central Plain. From Knock Eyon, a 

 limestone knob on the edge of Lough Deravaragh in Westmeath, 

 rising only 400 feet above the surrounding country, sixteen counties, 

 or half the counties of Ireland, may be easily numbered. Con- 

 versely, insignificant hills in the Central Plain become peculiarly 

 conspicuous. Croghan Hill in King's County, a granite volcano- 

 stump rising 500 feet from the plain, is a notable landmark over a 

 couple of thousand square miles around. Although the rivers are 

 generally slow-flowing, the area liable to inundation is not extensive. 

 Lakes are irregularly distributed ; and while some counties, such 

 as Westmeath, Roscommon, &c., have an abundant lake-flora, in 

 others, such as King's Co., Queen's Co., Kildare, were it not for 

 the canals, or for the rivers in Waterford and South Tipperary, 

 difficulty would be experienced in finding enough water to harbour 

 the common hydrophytes. 



