530 Mr. Archibald Geikic [Jiiuo 7, 



all through the chtiiu of the outer Hebrides, as well as on the 

 Archasau areas of the mainhmd. The most southerly points in 

 Scotland where it can be recognised are the island of iona and the 

 Eoss of Mull. It reappears, however, far to the south in Ireland ; 

 standing out in the bold cliffs from Erris Head to Achill Island in the 

 west of Mayo, and finally covering an area of more than 500 square 

 miles in south-western Gal way. In this last named district, as 

 Professor Hull has shown, so comj)letely are the scenic features of 

 the north-west of Scotland reproduced, down even to the minutest 

 details, that the geologist, even before he stands on the rocks, has no 

 difficulty in deciding that they can only be Archaean. 



What, then, are these most ancient rocks of north-western Europe, 

 and what has been their history ? Unfortunately the answer to these 

 questions cannot be succinctly and definitely given. Owing to the 

 antiquity of the masses, and the prolonged series of geological revolu- 

 tions which they have undergone, their original characters have been 

 somewhat effaced. In those areas where they have been least altered, 

 and where, therefore, they approach nearest to their primitive structure, 

 they have been found by my colleagues of the Geological Survey to 

 be crystalline rocks, such as gabbros, diorites, and other highly basic 

 comjiounds. These occur in zones or bosses surrounded by and 

 passing into rocks which have acquired the peculiarly banded struc- 

 ture characteristic of gneiss. That these various rocks were eruptive, 

 that is, that they originally formed portions of igneous material that 

 rose in a molten or plastic condition from below, can hardly be 

 doubted. They remind us of the deeiD-seated portions of some of the 

 eruptive bosses so abundantly intruded into the crust of the earth, and 

 now so ^plentifully exposed at the surface after prolonged denudation. 

 Like these, they show a rudely striped or banded arrangement 

 suggestive of the planes of movement or flow- structure seen in 

 consolidated igneous material. They have probably resulted from 

 successive protrusions of eruptive rocks at some depth within the 

 crust of the earth. 



Nowhere, however, in the region to which I am referring has any 

 trace of superficial eruption yet been detected. There are no true 

 volcanic ejections, nor any evidence that the rocks, though certainly 

 of eruptive origin, were ever connected with the ordinary explosive 

 operations of volcanic vents. Not only so, but after the most careful 

 search from Sutherland to Gal way not a vestige have we yet found 

 of any unquestionable sedimentary material. There are no con- 

 glomerates, no sandstones, no shales ; nor even any materials that 

 might be supposed to rej^resent these in a metamoriDhosed condition. 

 Of the actual surface of the earth these Archaean rocks afford no 

 recognisable trace. They obviously did not form the su^Derficial 

 layer themselves. They must have lain deep under a cover of other 

 material, under which they acquired their crystalline structure, and 

 by the subsequent removal of which they have been exposed to the 

 light. 



