August I, 1889] 



NATURE 



323 _ 



monly met with in the black shales of the Southern 

 Uplands of Scotland. The discovery of recognizable 

 fossils in these strata would fix the geological aoje of the 

 rocks of the Central Highlands 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 sedi- 

 mentary 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 north-east 

 are possibly to be traced even as far as the mountains 

 between the head of the Rivers Nairn and Findhorn, 

 where some of my colleagues think that there is probably 

 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 successful. Along the west of County Mayo, 

 Archaean gneiss has been recognized by us,- 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 direction of move- 

 ment, and its paste has been converted into a fine kind 

 of gneiss. 



Having thus traced an original westward boundary to 

 the younger crystalline schists of Ireland and Scotland, I 

 saw that it would be important to follow their eastern 

 boundary as far as it had not been concealed by later 

 formations. In Galvvay we found that the quartzites, 

 limestones, and schists are succeeded to the south by the 

 large area of Archaean gneiss already referred to. But 

 the boundary between the two groups of rock is one of 

 extreme complication, somewhat like that of the North- 

 West Highlands. Along a line running east and west 

 through the heart of this county from Mannin Bay to 

 Lough Corrib, the two groups have been so dislocated 

 and so thrust between and over each other that much time 

 and patience, with the use of large-scale maps, would be 

 required to map out their respective areas. But the im- 

 portant fact is readily perceptible that in Galway the up- 

 rise of a large Archaean area gives us a southern limit for 

 the basin in which the younger schists of the north-west 

 of Ireland were deposited. 



To the east and north-east of the Galway area the 

 country has been overspread with Old Red Sandstone 

 and Carboniferous strata, so that for a long space the 

 older rocks are concealed. Farto the north-east, in Tyrone, 

 on the southern borders of the great area of crystalline 

 schists, a mass of dark hornblendic rocks was mapped some 

 years ago by Mr. Nolan, of the Geological Survey oflreland, 

 and referred doubtfully to a pre-Cambrian age. A more 

 recent examination of this mass, with the experience 

 gained over so wide a region among the older crystalline 

 rocks, has enabled us to identify it without hesitation as a 

 characteristic portion of the Archaean gneiss. It rises as 

 a long north-east ridge along the south-eastern m irgin of 

 the chloritic schists of Londonderry which were deposited 



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

 obscure the evidence of the true relation of the rocks. 



In my recent traverses in the v/est of Ireland I had the advantage of the 

 company and assistance of my colleagues, Mr. Peich, Mr. Mc Henry, and 

 Dr. Hyland. 



against and over it. We discovered, moreover,*that these 

 schists have at their base, resting on the old gneiss, a 

 thick volcanic series consisting of amygdaloidal basic 

 lavas, tuffs, and coarse volcanic agglomerates. The 

 green chloritic material of the schists, not improbably re- 

 presents the original magnesian silicates in the finer 

 volcanic dust that mingled with the ordinary sediment of 

 the sea-bottom. 



From the evidence now adduced, it is, I think, manifest 

 that the crystalline schists of the Scottish Highlands east 

 of the Great Glen, as well as their continuation into the 

 north-west of Ireland, cannot be regarded as merely the 

 equivalents of the quartzites and limestones of Sutherland 

 and Ross. They are enormously thicker and more varied 

 in their component members than those north-western 

 strata. Whether even any part of them represents the 

 sedimentary rocks of the north-west seems to me open to 

 serious doubt. My own impression is that they are prob- 

 ably younger than these rocks, and that they once 

 stretched far to the north-west, and covered them to a 

 depth of many thousands of feet. That the fossiliferous 

 strata of the North- West Highlands were originally buried 

 under a thick pile of other sediments I have already 

 shown. 



The last question on which I propose to touch is 

 the geological date of the extraordinary terrestrial dis- 

 turbances to which the crystalline schists of the High- 

 lands of Scotland and the north-west of Ireland owe their 

 characteristic structures. The limit of its antiquity is 

 easily fixed. As these disturbances involve rocks con- 

 taining fossils of Lower Silurian age, they must obviously 

 have taken place after some part at least of the Lower 

 Silurian period. In Scotland their chronological limit in 

 the other direction is determined by the fact that the con- 

 glomerates of the Lower Old Red Sandstone are largely 

 composed of the crystalline schists of the Highlands. 

 They must consequently have occurred before the de- 

 position of some part at least of the Lower Old Red Sand- 

 stone. Here, then, is a long geological interval within 

 which the gigantic upthrusts and metamorphism began 

 and ended. 



But the evidence obtained in Ireland enables us to fill 

 up this interval with a little more definiteness. In 

 Southern Mayo and Northern Galway, as Prof. Hull has 

 pointed out, the Upper Silurian rocks rest upon and con- 

 tain abundant fragments of the younger crystalline schists 

 which stretch into these counties from Donegal. And 

 the inference has naturally been drawn that the great 

 terrestrial disturbances and metamorphism occurred 

 before the Upper Silurian period But a recent more 

 critical examination of the ground has satisfied me that 

 this inference, though to a certain extent correct, does not 

 embrace the whole truth. 



Those who have visited Connemara may remember the 

 singular group of mountains which hem in the Killary 

 fjords, and rise in Mweelrea and its neighbouring ridges 

 to a height of more than 2600 feet above the sea that frets 

 their base. These massive buttresses of rock owe their 

 distinctive forms to the thick beds of coarse grit and con- 

 glomerate of which they are in great measure built up. 

 An abundant series of fossils proves that this mass of 

 deposits is of Upper Silurian age. It is the base of these 

 exceedingly coarse sediments which along their southern 

 margin can be seen to rest upon the upturned edges of 

 the crystalline schists, and to be there largely made up of 

 fragments derived from that metamorphic platform. The 

 numerous bands of coarse conglomerate upon successive 

 horizons serve to indicate considerable terrestrial dis- 

 turbance during their deposition. That the commotion 

 continued after that time is further shown by the 

 remarkable way in which the rocks have been dislocated. 

 These Upper Silurian sediments have been broken up 

 into large mountainous blocks which have been thrown 

 on end or actually pushed over each other. So violent 



