506 O. S. Kinahan — Archcean Rocks. 



are the equivalents of the Arenig (Passage Beds between the Ordo- 

 vician and Cambrian) and part of the Cambrian ; the lower portion 

 of the Ordovician (Barh shale series) being absent. And if the 

 markings near Fintown, Co. Donegal, in the black shales belonging 

 to the rocks below the nnconformability (gneiss and schist series) , which 

 were exhibited at the British Association Meeting, Birmingham 

 (1886), turn out to be Graptolites of Arenig types, as some consider 

 them, such evidence ought to prove the correctness of this suggestion. 

 This classification originally was made from petrological or strati- 

 graphical evidence. 



The Gal way area, declared to be of Arch^an age, is as follows. 

 To the south-east granitic-gneiss with metamorphic and intrusive 

 granite ; these are margined by gneiss and schist ; while in the 

 western portion of the area there are rocks, principally argillaceous, 

 very slightly altered, with intrudes of granite. The south-east 

 granitic-gneiss can be traced, first northward, then north-westward, 

 through schist into the " Doolough bed," in the neighbourhood of 

 Killary Harbour; the latter rocks having in them fossils of Llandeilo 

 types ; while westward this granitic-gneiss can be traced through 

 schist into the slightly-altered rocks margining the western coast. 

 The latter rocks have all the characteristics of the " Doolough beds," 

 although fossils have not as yet been found in them. 



The supposed boundary of the Archasan area is an arbitrary line 

 drawn nearly westward from Clifden to Oughterard ; while both 

 north and south of this line the major portion of the rocks are 

 lithologically identical. Therefore there are no records of a vast lapse 

 of time. Stratigraphically it seems proved, that the so-called Archceans 

 of Qalway, instead of being the oldest rocJcs of the country, are tlie 

 equivalents of the latest Ordovicians. 



"In North-west Mayo (Erris) the tract that has been given a brevet 

 rank is a patch of granite and gneiss, in sub-metamorphic rocks. Here 

 there is a marked distinction between the lithological character of 

 the rocks ; but there are no records of a lapse of time, as there is 

 no basal conglomerate in connection with the sub-metamorphic 

 rocks, which the description given in the Geological Survey Memoir 

 has led some to believe. The rocks in this tract have undergone 

 much more metamorphism than those in the adjoining county, 

 which led Griffith to believe they were older ; possibly they may be 

 of Archaean age — or the still-unnamed Passage Beds ; but it seems 

 more probable that they are of the same age as the rocks in North- 

 east Mayo and South Donegal, which are as much, if not more altered, 

 and whose age probably is either Arenig or Cambrian. 



In North-east Mayo, Sligo, Leitrim, and Donegal, there are no 

 hard boundaries to the tracts of the so-called Archseans ; as the 

 rocks in general graduate one into another, except in a few 

 places where faults or intrudes occur. So gradual is the change, 

 that the supposed boundaries have several times been changed. In 

 fact, they are like the rolling fences of a farm adjoining a common, 

 one time here and another time a quarter of a mile or more away ; 

 yet, in reference to the Archsean boundary, each successive line is 



