1880.] on the Highlands of Scotland and the West of Ireland. 513 



land and Ireland, as a necessary foundation for placing them on their 

 true geological horizon, and at length this spring our efforts have 

 been successful beyond our expectations. Last year in the north-west 

 of the island of Islay I found a group of scarcely altered shales, 

 grits, and thin limestones emerging from beneath the black slates 

 which underlie the schists, limestones, and quartzites of that region. 

 So little have these strata suffered from the metamorphism which has 

 affected the rocks lying above and to the east of them, that I quite 

 anticipate that fossils will be found in them. This year, in company 

 with Mr. C. T. Clough, I came upon a somewhat similar group of 

 little metamorphosed black slates and grits at the north-east end of the 

 island of lona. I am hopeful that these strata will yield fossils ; I 

 myself found in them some short black lines, which at once recalled 

 the form and condition of the fragments of the central rods of grapto- 

 lites so commonly met with in the black shales of the Southern 

 Uplands of Scotland. The discovery of recognisable fossils in these 

 strata would fix the geological age of the rocks of the Central High- 

 lands and of the north-west of Ireland. 



An interesting feature about these slates of lona is that they lie 

 at the very bottom of the series of younger schists. Immediately 

 under them are a coarse grit (arkose) and conglomerate, formed out of 

 the Archaean gneiss, which comes out in great force from underneath 

 them and forms the main part of the island.* The uprise of an axis 

 of the old gneiss so far to the east of the line of great complication, 

 and at the base of the vast sedimentary masses of the Central 

 Highlands, is a fact of great importance. We seem to find, here a 

 fragment of the old barrier which separated the American province 

 in which the Durness limestones were deposited, from the area of 

 Western and Central Europe in which the other Silurian formations 

 of Britain were laid down. Prolongations of the same ridge towards 

 the nortli-east are possibly to be traced even as far as the mountains 

 between the head of the rivers Xairn and Findhorn, where some of 

 my colleagues think that there is j^robably another core of the 

 Archaean gneiss. 



The search for a base to the same great series of schists as they 

 are developed in the north-west of Ireland has been equally success- 

 ful. Along the west of County Mayo Archaean gneiss has been 

 recognised by us.f exhibiting the typical characters of the same rock 

 in the north-west of Scotland. In Achill Island we found the 

 base of the quartzite and schist series in the form of a coarse quartz- 

 conglomerate resting on the gneiss. But all these rocks have come 

 within the influence of the intense regional metamorphism. The 

 conglomerate in particular has had its quartz-pebbles pulled out in 



* The existence of a slight displacement at the actual junction does not 

 obscure the evidence of the true relation of the rocks. 



t In my recent traverses in the west of Ireland I hail the advantage of the 

 company and assistance of my colleagues, Mr. Peach. Mr. M'Henrv. and 

 Dr. Hyland. 



