*9°7- Seymour.— Gwlogy of Lam bay. n 



from their original level, which must have been out of reach 

 of material derived from the local rocks. Of course an 

 obvious explanation would be that the igneous rocks are later 

 than the Old Red Sandstone in age and that the latter have 

 been uptilted b)' the intrusion of the former. However the 

 weight of evidence is undoubtedly in favour of the pre Old- 

 Red-Sandstone age of the igneous rocks. 



The Igneous Rocks and Ashes. — The greater portion of 

 Lambay is made up of an andesitic rock possessing the usual 

 characteristics of that type. Phenocrysts of plagioclase 

 felspar are noticeable in thin sections, but are inconspicuous 

 in the hand specimen. Usually the rock is altered somewhat 

 in regard to many of its original constituents, and a good 

 deal of secondary chlorite and calcite is developed, and also 

 some epidote. The ferro-magnesiau mineral is usually augite, 

 which sometimes becomes dominant, and the rock then 

 passes over into the augite-andesite group, the [distribu- 

 tion of which rock on the island will be referred to later on. 

 A rhombic pyroxene (hypersthene) is developed in the andesite 

 next the limestone at Kiln Point. Though generally compact 

 in type, examples of amygdaloidal andesite occur (N.E. of 

 Bishop's Bay), and "xenolithic andesites" occur locally. These 

 latter rocks are andesites containing more or less angular 

 fragments, probably blown on to the surface of the lavas from 

 a neighbouring volcanic vent, and gradually incorporated in 

 the slowly moving lava flow. 



An andesite, in which the dominating phenocrysts are augite 

 crystals, occurs in several isolated localities on the island. 

 A prominent outcrop is that on L,ambay Head (Plate 2), where 

 it forms the backbone of that portion of the island. Some of 

 the augites are of considerable size, and are fairly fresh, 

 though the felspars are almost entirely altered to secondary 

 products. Augite andesites also occur near Raven's Rock, 

 Knockbaue, and some 400 yards west of Flint Rock, and in a 

 few minute outcrops elsewhere. In microscopic section this 

 type of rock shows an altered ground-mass of felted felspar 

 microlites (referable to andesine and labradorite), augite 

 granules, and magnetite, with secondary hematite and chlorite 

 as decomposition products. The phenocrysts are chiefly 

 fairly fresh augites, showing characteristic cleavages and 



