278 DE. C. A. MATEEY ON THE [May 1906, 



but I hope to present an account of the remainder of the section in 

 a future paper. 



The description of these beds in the Explanatory Memoir of the 

 Geological Survey of Ireland (Sheets 102 & 112) l seems to be the 

 only detailed account that has hitherto been published. In G. V. 

 l)u Dover's section on p. 62 of that memoir, all the beds at the Rush 

 end of the coast-section are lettered d4, except a small patch of black 

 shale south of Brook's End which is marked cl5, and this notation 

 agrees with that in the 1-inch Geological Survey-map (Sheet 102, 

 revised 1901); but d5 is absent from the index of the map, and 

 the beds so marked are coloured as though they were the ' dS & 

 dJf (Millstone Grit & Yoredale Beds)' of the index; while, to add 

 to the difficulty of reading the map, the d4 areas are coloured to 

 correspond with the ' d 2 (Calp) ' of the index. It may be presumed, 

 however, that the colouring, and not the lettering, indicates the 

 current opinion of the officers of the Survey as to the age of the 

 beds at Rush. 2 



The term Calp is used by the Geological Survey to include in 

 this particular area both the true Calp (Middle Limestone) and the 

 Upper Limestone, as the latter is believed to have taken on a 

 ' calpy ' character here, and thereby to have become indistinguish- 

 able lithologically from the Middle Limestone. 



Palaeontology has hitherto been of little service in separating the 

 beds, but, as will be shown below, Dr. Yaughan has now ascertained, 

 from an examination of the fossils of the Rush sequence, that the 

 same faunal succession is to be found, with slight modification, 

 on both sides of St. George's Channel, and he has been able to 

 subdivide the beds into palasontological zoues corresponding with 

 those that he has already described from England and Wales. 



II. General Structure and Sequence. [C. A. M.] 



The Carboniferous beds of Rush are separated on the south, from 

 the Ordovician and Old-Red-Sandstone inlier of Portraine y by the 

 mouth of an estuary 1| miles wide, and on the south-east, from the 

 somewhat similar rocks of Lambay Island 3 by 2| miles of sea. 



1 2nd ed. (1875) pp. 61-66. 



2 On my making enquiries of Prof. Grenville Cole, F.G.S., who now has charge 

 of the Irish Geological Survey, he has heen good enough to explain how the con- 

 fusion between lettering and colouring has come about. It appears that, when 

 Sheet 102 was originally issued in 1859, certain outliers of 'Coal-Measures' 

 (the Poddonomya-Eeds) were coloured on it in the Rush area. When revised 

 about 1875 the ' Calp ' anticlinals along the coast were inserted, but without 

 the removal of the letter d 4- In the index the Calp is marked d3 & d 4, and 

 the Upper Limestone d4 also. At a later revision the 'Coal-Measure' area 

 was coloured 'Millstone Grit & Yoredale,' and some of the d 5 marks were 

 altered to d3, the margin showing 'Millstone Grit & Yoredale Beds' as dS 

 & d4- But d4 was allowed to remain on the Calp anticlinals, which now 

 should have been lettered d:2. Prof. Cole adds that it is intended to rectify 

 the discrepancies in the map at an early date. 



3 For an account of the rocks of Portraine and Lambay Island see C. I. Gai-- 

 diner & S. H. Reynolds, Quart, Journ. Geol. Soc. vol. liii (1897) p. 520, & 

 vol. liv (1898) p. 135. 



