1907. Skymour. — Geology of Lambay. 9 



occur, but the largest number of these were obtained by the 

 writer many years ago from the material excavated by rabbits 

 burrowing in some low mounds in the field east of Carnoon 

 Bay, and recorded in the Irish Naturalist ( 7 ). Most of the 

 specimens were coarse rough chips, but several cores were 

 found, also one chip showing secondary working. 



Alluvium. — This deposit is of only very limited extent, being 

 confined to a small area just above Seal Hole, and again in a 

 hollow or two along the course of the streamlet flowing into 

 Freshwater Bay. The material is a dark moory or peaty clay 

 formed by the decay of vegetable matter mixed with a little 

 clay in some hollows, kept continuously moist by drainage 

 water from springs and the surrounding high ground. 



Solid Geology. 



The various rocks which go to build up the island are 

 divisible into two main groups — those of sedimentary origin 

 and those formed by igneous activity. The former comprise 

 shales and limestones of Silurian origin, also a peculiar lime- 

 stone conglomerate betweeu Kiln Point and Seal Hole. In 

 addition, a small area on the south side of Broad Bay consists 

 of coarse conglomerates and red shales referable to the Old 

 Red Sandstone period. The igneous rocks, which make up 

 by far the larger portion of the island, not less in fact than 

 19/20 of the whole, are andesitic in type. The rock most 

 prominently developed is a fine non-porph} T ritic andesite, 

 while andesitic ashes, porphyritic andesite, and an augite 

 andesite occur in subordinate amount, and in small outcrops 

 scattered over the island. 



Silurian sedimeiitary rocks. — The main mass of shales 

 occurs on the east side of the island, and forms the high 

 ground between the summit of Heath Hill down to the shore 

 line. These rocks are bounded by a fault on the north which 

 may be observed in the cliff above Seal Hole. The rocks are 

 olive-brown fissile shales much jointed, the joints being usually 

 coated with a bronzy ferro-manganese deposit. Their 

 relations with the limestone at Kiln Point (Plate 4) is not very 

 clear, but the latter appears to be banked against them after the 

 manner of a coral reef. This limestone shows signs of altera- 

 tion by the andesite, the change being noticeable both in the 

 hand specimen and under the microscope. The corals found 



