116 Scientific Proceedings^ Royal Dublin Society. 



is found only in the cracks and hollows and on the bottoms of shallow pools ; 

 whereas Vemicaria maura grows best on rocks fully exposed to sun and air 

 and can endure drought for many successive days. 



ii. Calcareous Rocks. 



The Carboniferous limestones which overlap the Cambrian rocks at the 

 western end of the peninsula stretch from Sutton to Balscadden Bay. On 

 the Sutton side, though they reach the shore, they are almost entirely 

 obscured by the Boulder-clay ; but near the village of Howth on the north 

 coast they are exposed in the east corner of the Harbour, and on the west of 

 Balscadden Bay, where they form a long, low-lying stretch of foreshore 

 with some higher strata about high-tide mark. Plate VIII, taken at 

 about half-tide, shows the lie of the rocks. Behind them are the stratified 

 drift-banks on which Balscadden Terrace is built. The waves wash the base 

 of these banks during high spring-tides. The limestones are mainly rather 

 soft and light-coloured ; but here and there tracts of a dark brown limestone, 

 harder in texture and dolomitic in character, are mixed with them. 



The lichens so common on the Cambrian strata are almost absent from the 

 limestones ; where they occur it is only as thin, poorly developed patches. 

 How far this is due to the nature of the rock or to other causes it is not easy 

 to say without an examination of a larger area of limestone shore ; but it 

 seems probable that the composition of the rock has some effect on the lichen 

 growths, as three species not found on the silicious rocks are the prevailing 

 forms on the limestone. 



Til ere are no Kamaliuas ; but this is most likely due to the absence of 

 high rocks and to the situation being close to habitations. The orange lichens 

 are very poor, a few spots of Placodium calopisuia and Lecanora citrina being 

 the only representatives. Associated with them were scattered colonies of 

 HMzocarjwn alboatrum, Rvnodina exigua var. demissa, and Lecanora galactina. 



Verrucaria maura, and all species with gelatinous thalli, with the exception 

 of a few spots of V. aquatilis, which grew in the weathered hollows on tlie 

 tops of the rocks above high-water mark, were entirely absent above half- 

 tide. 



The algal growths, too, seemed to begin at lower levels than on the 

 Cambrian shores. They were, first, a narrow band of Fucus vesiculosus. Below 

 this was a wide and very fine bed of F. serratus, which lower passed into an 

 equally luxuriant growth of Lamiuarias (L. saccharina and L. digitata)^ 

 L. saccharina rising higher on the shore in the troughs and hollows of the 

 rocks. The tops of the rocks near low water being covered in many places 



