522 Scientific Proceedings, Royal Dublin Society. 



adjoining the shore-line, respectively, being generally of reddish or purplish colours, 

 and more or less coarse, often conglomerates. But not always so, as sometimes they 

 are fine red shales. Above these, or farther out from the shore, the arenaceous rocks 

 become yellow and grey sandstone, with more or less subordinate grey and bluish 

 shales. This graduation generally takes place upwards, but not always ; as in Galway 

 and Mayo, near Oughterard and Castlebar, you can trace, along the strike of the 

 bedding, conglomerates graduating into sandstones, and the latter into pebbly lime- 

 stones. This also can be seen in various other places, as between Ballyshannon and 

 Pettigoe, Counties Donegal and Fermanagh. Griffith was aware that sandstones 

 of both these colours and textures were the basal beds, or "shore beds," of the 

 Carboniferous limestone; but, to meet the nomenclature of the day, he called the 

 dark-coloured rocks " Old Bed Sandstone,'''' and forthe light-coloured he introduced the 

 term " Yellow Sandstone." Jukes, however, adopted a different course, as he included 

 both together in his Upper Old Red Sandstone. 



Of late years this merely lithological distinction has again, in places, been intro- 

 duced and given an unnatural value ; so that we find on the new maps little spots 

 called " basins of Old Bed Sandstone," solely because the rocks are of dark colour and 

 coarser texture, while in other places exactly similar rocks are given their natural 

 place : that is, they are grouped as the basal or shore beds of the Lower Limestone. 

 In Western Mayo the rocks are placed in their true position ; but this has not 

 been done in Eastern Mayo, although, as pointed out by Symes, the classification 

 into two distinct formations is "chiefly lithological" (Geological Survey Memoirs, 

 sheets 41, 53, and 64, page 14, and footnote by Dr. Hull). From the description 

 of the rocks of "Western Mayo it will be seen that, similarly as Griffith mapped 

 them, these ought also to be "Old Bed Sandstone" in the eastern area: that is, if 

 there is " Old Bed" in the east of the county, it must also occur in the west, if the 

 lithological character had been given the same value in both districts (Geol. Mem., sheets 

 39, 40, 51, 52, and 62, page 16). Griffith, and subsequently Jukes, were gradually 

 bringing Irish geology out from the mists of the past, and it seems regrettable that it 

 should now be plunged back again into the dark ages.] 



The fauna of the lower group (Lower Carboniferous Sandstone or 

 Yelloiv Sandstone), although it was unsuited for the clearer and 

 deeper waters in which the associated limestones accumulated, did 

 not die out, but emigrated to the congenial littoral shallow waters, 

 afterwards to again spread out in later times (Calp), when the 

 accumulations and conditions were favourable. Thus, we find in 

 the Lower Carboniferous] sandstones and shales, in the Littoral 

 sandstones and shales, and in the Calp accumulations, that the 

 rocks and their fauna are more or less similar. There is, how- 

 ever, in places in the Calp, a marked change in the accumulations, 

 they being more or less calcareous, and even in places good lime- 

 stone. Yet it is remarkable that in them, as in the shaly lime- 

 stone of the Rathkeale district, Co. Limerick, the assemblage of the 

 fossils is very similar to that of the Lower Carboniferous sandstone, 

 in both being found many forms which are not to be met with in 



