12 THE JOURNAL OF GEOLOGY. 



There can be little doubt that a strong unconformability exists 

 between them. A close examination of the ridge of old gneiss 

 in Tyrone and Fermanagh showed me that though the actual 

 basement-beds of this Dalradian series could not be seen resting 

 on the coarse gneiss, the lithological character, and tectonic 

 arrangement of this series are only explicable on the supposition 

 of a complete discordance between it and the gneiss. As these 

 two groups of rock have never been found in close proximity in 

 Scotland, and as the determination of the true age of the 

 Dalradian series is a question of such great stratigraphical 

 importance in the general mapping of the United Kingdom, I 

 requested Mr. A. McHenry, of the Geological Survey of Ireland, 

 to continue the tracing of the mutual boundaries of the old 

 gneiss of the Ox Mountains and the Dalradian series in County 

 Mayo. He informs me that he has found in that series a con- 

 glomerate full of blocks of the old gneiss, and resting in one 

 locality apparently unconformably upon it. If this observation 

 is confirmed it will finally set at rest the relative position of the 

 coarse massive gneiss and some portion, at least, of the 

 Dalradian series. Of course there is no absolute proof that the 

 coarse gneisses of Ireland are really the equivalents of the 

 Lewisian masses which they so closely resemble. But there is 

 a strong presumption in favor of their identity. 



In England and Wales many detached areas of rock have 

 been claimed as pre-Cambrian, and successive formations have 

 been classified among them. I have already dealt in part with 

 this question, and without attempting here to review the volum- 

 inous literature of the subject, I will content myself with stating 

 briefly what seems to me to have been established on good 

 evidence. 



There can not, I think, be now any doubt that small tracts of 

 gneiss, quite comparable in lithological character to portions of 

 the Lewisian rocks of the north-west of Scotland, rise to the 

 surface in a few places in England and Wales. In the heart of 

 Anglesey, for example, a tract of such rocks presents some 

 strikinof external or scenic resemblance to the characteristic 



