174 Succession of the Older' Stratified B,ocks in Ireland. 



" that all those tracts which occur between Dublin and Dun- 

 dalk, along the course of the Boyne, and in the hills separa- 

 ting tlie counties of Cavan and Meath, which have been de- 

 scribed as the older graywacke or transition series by Mr. 

 Griffith, Mr. Weaver, and others, are in reality all conform- 

 able, and immediately inferior to the mourftain limestone and 

 superior to the old red sandstone, and consequently belong 

 to the Devonian series." 



I confess I am surprised at the view here taken by Mr. 

 Hamilton, as one of the facts on which his argument is 

 grounded, namely, that the rocks coloured by me as transition 

 are superior to the old red sandstone, have been correctly 

 stated by himself* to be inferior to that rock where it occurs 

 near Balriggan mill, N.W. of Dundalk, in the county of 

 Louth, in which locality the old red sandstone rests uncon- 

 formably on the transition slate. 



In regard to the second point, namely, that the schistose 

 rocks are succeeded by the limestone in a conformable posi- 

 tion, I have to observe that such is not the fact ; for in the 

 only localities in which I have been hitherto enabled to ob- 

 serve the contact of the two rocks, the limestone rests uncon- 

 formably on the transition slate. These localities are in 

 the river north-east of the Naul, in the county of Dublin ; 

 at the southern extremity of the village of Duleek, in the 

 county of Meath ; at Old Bridge on the banks of the Boyne, 

 two miles west of Drogheda, in the county of Louth ; and at 

 Headfort near Kello, in the county of Meath. 



Fortunately, in addition to these facts, we have also another, 

 which is quite conclusive, namely, the discovery of fossils 

 belonging to the lower Silurian rocks or Caradoc sandstone, 

 which occur in considerable abundance at Grangegeeth, four 

 miles north of Slane, in the county of Meath : the fossils have 

 recently been examined by Mr. Murchison and Mr. Lonsdale, 

 of the Geological Society of London, to whom 1 had sent 

 them, and both are of opinion that they belong to the lower 

 Silurian rocks f. Consequently we must come to the conclu- 

 sion that Mr. Hamilton's opinion is erroneous in respect to 

 the geological position of the slate district, north of Dublin, 

 and of that between Drogheda and Dundalk. 



I have now replied to all the important observations con- 

 tained in Mr. Hamilton's paper, which tend to cast a doubt on 

 the accuracy of my Geological map, and I think I have been 



* See Journal of the Geological Society of Dublin, vol. ii. part i. 



f The fossils found, and which have been compared with the original 

 Silurian forms collected by Mr. Murchison, are Orthis semicircularis and 

 Orthis virgata; in addition to which there are several other forms of the 

 genus Orthis which have not as yet been clearly identified, 



