Trap-Byhes of Mayo, IrelancL 559 



The lowest or basal bed of the Carboniferous series is a con- 

 glomerate of from one to four feet in thickness, composed chiefly 

 of fragmental vein quartz and pieces of the Metamorphic rocks ; it 

 merges imperceptibly into the overlying sandstone beds, and there 

 is no break of continuity. In the geological map referred to, there 

 ■was represented a band of Devonian rocks intermediate between the 

 others, which he (the author) in all due deference called in question, 

 and which he believed did not exist, the thin conglomerate bed un- 

 doubtedly belonging to the Carboniferous series. 



The intrusive igneous masses which so abundantly penetrated the 

 rocks of the district, as Trap-dykes, occur in both the Metamorphic 

 and Carboniferous areas, but more abundantly within the former, and 

 belong to very different ages ; the one set being Pre-Carboniferous, 

 and the other Post-Carboniferous, and possibly of Tertiary (Miocene) 

 age. 



Both classes of dyke may be considered as primarily belonging to 

 the basaltic type, but the one seems in part at least to have been 

 considerably altered by subsequent metamorphic actions. Both sets 

 penetrate the older or Metamorphic rocks, but only the second set 

 the Carboniferous beds. 



Though in many respects closely resembling each other, there are 

 certain characteristic distinctions peculiar to each, specially the 

 peculiar weathering of the older dykes, which on the exposed sur- 

 faces showed a crystalline white mottling of the weathered felspar 

 crystals through the dark green base. 



The junction of the Carboniferous with the Metamorphic rocks 

 being mostly obscured, the direct evidence of the older dykes ending 

 off at that junction was not obtainable, and not finding any in situ in 

 the Carboniferous area was only negative evidence ; but after long 

 search, undoubted fragments of those characteristic older dykes were 

 found in the conglomerate or basal bed of the Carboniferous rocks, 

 thus proving unquestionably the Pre-Carboniferous age of those dykes. 



The older dykes occur mostly in large sheets from 10 to 150 feet 

 in thickness, and are approximately interbedded among the Meta- 

 morphic strata, and often contorted with them, though often them- 

 selves the cause of the minor crumplings ; occasionally they are 

 folded back on themselves, forming large S curves. 



Instances of these were described and specimens exhibited, from 

 Benmore, and from Belderg Harbour, where the exposed part seems 

 to be the top surface of the sheet, consisting of large humps and 

 bosses protruding through the quartzites and schistose beds. In this 

 typical example, the effects of the metamorphic action at different 

 depths was well shown, the central part being a hard, heavy, and 

 splintery dark steel-blue, micro-crystalline basalt, passing into a 

 fibrous hornblendic schistose rock, with soft green chlorite and nests 

 of green hexagonal chloritic mica, and towards the exterior becoming 

 very schistose, platy, and micaceous, resembling a black or bronze 

 mica-schist ; in part these might be considered as Diabases. The 

 felspar seems to be plagioclastic or triclinic, and on the weathered 



