1907. Seymour. — Geology of Lamb ay. 7 



accompanying extrusion of lavas and ashes, and occasional 

 intrusive masses of porphyry. On the new land surface, 

 probably in inland lakes, deposits of Old Red sandstone and 

 conglomerate were ultimately formed, the whole series under- 

 going a tilting and faulting later on, bringing them into their 

 present relations to one another. In early Carboniferous 

 times the land sank beneath the sea, and bed after bed of 

 shales, limestones and grits was deposited on the older rocks, 

 to be subsequently removed after ages of denudation, and the 

 old land surface again appearing was moulded to its present 

 form by the slow action of Nature's graving tools, and the 

 smoothing action of an ice-sheet. 



The Drift Deposits. 



The superficial deposits occurring on Lambay are divisible 

 into four groups, only two of which occupy any considerable 

 area : — Boulder-clay, local drift, raised beach, and alluvium. 



Boulder-clay. — This is the oldest form of drift on the island, 

 and chiefly occupies the lower ground on the west side of the 

 island, and also the chief valleys previously mentioned (p. 5). 

 Small masses of the same deposit occur plastered against the 

 steep cliff slopes on the east side of the island, these being 

 generally covered with a sandy detritus, derived from the de- 

 cay of the rocks in the vicinity. Towards the hill slopes the 

 Boulder-clay becomes somewhat modified by the partial incor- 

 poration of local detritus, passing gradually into the latter 

 material when close to the rock floor. Good sections of 

 Boulder- clay are few and far between on the island, the best 

 being exposed on top of the Old Red Sandstone at Broad Bay. 

 This section is some six feet thick, and contains scratched 

 pebbles of limestone, and others of sandstone, chert, flint, and 

 andesites, with some odd shell fragments of Tellina(?) and 

 Astarte sulcata (?) Smaller sections of typical tough limestone 

 boulder-clay may be seen in the cliff face north of Seal Hole, 

 and again to the south and east of Bell Rock. In general the 

 boulder-clay areas are characterised by smooth and gently 

 sloping grass-grown ground, rather sharply differentiated from 

 the bracken and heather clad areas with, practically, no depth 

 of soil. 



