1907. 3 



GEOLOGY. 



BY HENRY J. SEYMOUR, B.A., F.G.S. 



The island of Lambay lies in the Irish Sea off the coast of 

 Co. Dublin, in N. lat. 53 29', W. long. 6° 1'. It is roundish 

 in outline, well raised above sea-level, and has an area of 617 

 acres above high water mark. The channel which separated 

 it from the mainland is 2% to 3J miles wide, and maintains a 

 tolerably uniform depth of six to seven fathoms. At flow and 

 ebb, a strong tide sets through this channel, the rise of tide 

 being 13 feet at springs, 10 feet at neaps. Shoal water extends 

 north and south of the west end of the island for several miles, 

 tending to define the channel more strongly. Outside (east) of 

 Lambay, the water deepens rapidly into the Irish Sea basin. 



The climate of Lambay is thoroughly insular in character — 

 cool in summer, warm in winter. Frost and snow are rare. 

 During the year 1906 (a period of mild winters) frost was re- 

 gistered on 13 nights, the lowest temperature being 25 F. on 

 December 28 and 29. On only one occasion (December 29) 

 did the thermometer register below 32° F. at 10 a.m. The 

 climate is also a dry one for Ireland, Lambay- lying within the 

 area of least rainfall in the country — a strip of the east coast, 

 with a precipitation of under 30 inches. 



Previous L it era tu re. 



The first detailed account of the geology of the island 

 appeared in the year 1861, in the Geological Survey Memoir to 

 accompany the one-inch sheets Nos. 102 and 112. The island 

 was surveyed by the late G. V. Du Noyei^ 1 ), and the descrip- 

 tion written by him in conjunction with the late J. B. Jukes, 

 F.R.S., the local director of the Irish Survey at that time. 

 The occurrence of the well-known "Lambay porphyry,' 5 a 

 rock similar to the " verde antique " of ancient writers, has 

 been dealt with and described by Von Lasaulx( 3 ) in 1878, and 

 subsequently by Teall( 4 ) and Harker( 6 ) in their works, pub- 

 lished in 1888 and 1895 respectively. Professor Hull ( 2 ) 

 also described the Lambay porphyry, so far as the then 

 immature condition of the science of petrography permitted, 



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A 2 



