KiNAHAN — On Supposed Upper Cambrian Hocks. 345 



I think, are probably equivalents of part of the " Arenig group" of 

 Wales, put by some geologists among the Cambrians. 



These two lines of evidence, stratigraphical and petrological, 

 point to the same conclusion, that these Tyrone rocks now in question 

 are of Cambrian age. 



It may here be mentioned incidentally, that breaking up through 

 these metamorphic eruptive and sedimentary rocks are masses of a much 

 more recent granite, or elvan, similar to the pre-Carboniferous granite 

 rocks at Carndaisy, Co. Derry, and more or less like the granite and 

 elvan of Lugnanoon, Co. Galway. As this granite, in the intrusion 

 near Mullanmore Bridge (north of Carrickmore, or Termon Rock), 

 graduates into elvan, and the latter into a quartzitic eurite, closely 

 allied in composition to the eurites which, south of Pomeroy and 

 elsewhere, are interstratified with the arenaceous red Silurian rocks, 

 we have a right to suppose that here, as in "West Gralway, and South- 

 west Mayo, these granites and elvans are the granitic roots of the 

 Silurian bedded eruptive rocks. 



The second tract to which I would direct attention is in I^orth- 

 east Mayo, southward of Charlestown, and westward of Ballaghader- 

 reen. The rocks of that area were classed by Griffith, and subsequently 

 by Jukes, as ancient metamorphic rocks ; but more recently they have 

 been called " Felstones" and " Upper Silurian." 



Mapping some of these rocks as felstones is so far correct, that 

 the rocks so called are granulites, or leptynites (metamorphic fel- 

 stone), rocks which many petrologists include among the felstones ;^ 

 but mapping any of them as " Upper Silurian" must be incorrect, as 

 it is evident that these metamorphic rocks were ruptured, upturned, 

 metamorphosed, and denuded prior to the still unaltered Silurian 

 rocks being deposited on them. In the conglomeritic Silurian rocks 

 of Cranmore and elsewhere fragments of these metamorphic rocks are 

 conspicuous. 



These metamorphic rocks in the country southward of Charles- 

 town are in many respects very like those in the Tyrone hills to the 

 north of Pomeroy ; but we cannot speak of them so confidently, 

 because — 



First, only a small tract is exposed, which at one side is overlaid 

 unconformably by Silurian rocks, and on the other by Carboniferous ; 

 therefore, although we are aware that they must be older than the 

 Silurian, yet we cannot positively prove that they do not belong to 

 the Cambro-Silurian. 



Secondly, in many respects they are more or less similar to some of 

 the rocks of the Doolough and Lettermullen series (S. "W. Mayo and 

 "West Galway), in which are fossils pronounced to be Cambro- 

 Silurian. 



* Rutley says that graniilite, or leptjnaite, has the same relation to felstone as 

 gneiss has to granite. 



R. I. A. I'EOC, SEK. II,, VOL. III. — SCIENCE. 2 D 



