lo The Irish Naturalist. Jauuary, 



The Areiiig grits are replaced north of the harbour by a 

 wide spreading mass of highly cleaved, green slates (the 

 Doolough vSlates) with subordinate grit bands. Only where 

 the cleavage coincides with the bedding can identifiable 

 fossils be obtained. These slates, at the north end of 

 Doolough, have yielded Diplograpliis dentatus, Phyllograptus 

 (4 species), Logaiwgraptus, Tctragraptus^ Trigonograptus^ and 

 other forms, the majorit}" of which are now found in Ireland 

 for the first time. At other localities along the shores of 

 Doolough, Phytlograpths and Didymogi-aptus extensus were 

 obtained. The Doolough Slate group seems therefore to be 

 of Upper Arenig age, and on the same general horizon as the 

 Leenane and Rossroe Grits. 



Following on the Doolough Slates in conformable succession 

 are the Mweelrea Grits. These current-bedded, red, and 

 greyish-green, felspathic grits form a remarkable feature of 

 the district. The lower 3,000 feet contain bands of green 

 shale, yielding abundant fossils locally. Graptolites are 

 absent, but brachiopods are plentiful, and at one spot the 

 characteristic IJandeilo trilobite Ogygia was found. Kast of 

 Killar}' Harbour these grits attain an enormous development on 

 the Formnamore plateau. Here they seem to be upwards of 

 12,000 feet thick, the upper 9,000 feet being one monotonous 

 mass of felspathic grit, without a single parting of shale, and 

 quite destitute of fossils. In view of their thickness, these 

 grits may, perhaps, include strata of Bala age. We have no 

 definite evidence of the existence of Bala beds in the district, 

 since the Silurian rocks do not rest on the Mweelrea Grits, 

 but on the Cryf-talline Schists. 



The suggestion is made that a considerable part of the 

 Mweelrea Grits, containing as they do an abundance of angular 

 grains of fresh, pink felspar, and exhibiting a conspicuous 

 false-bedding and lenticular pebble-beds, were accumulated 

 on a land-surface under arid continental conditions. In any 

 case their deposition was followed by a period of prolonged 

 erosion, during which a great thickness of Ordovician rocks 

 was removed from the Connemara Crystalline Schists, so that 

 the Silurian rocks now rest unconformably on the schists along 

 a line running eastwards from Gowlaun on the coast along 

 the south side of I,ough Fee to the head of the Maam valley. 

 At the base is an irregular breccia, consisting of fragments 



