THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE V. VOL. VIII. 



No. III. — MARCH, 1911. 



OIRIGKT.IN'.A.JLi AETICLES. 



I. — On a Preglacial Shoreline in the Western Isles of Scotland. 

 By W. B. Wright, F.G.S. 

 (PLATE VI.) 

 ri^HE preglacial x platform of marine erosion discussed in the 

 J_ following pages was first discovered in the Island of Colonsay 

 in the course of the geological survey of that island carried out in the 

 summer of 1907, and the information then acquired is communicated 

 with the permission of the Director of the Survey. The subsequent 

 tracing of the shoreline in the adjoining islands was effected with the 

 aid of a Government grant for scientific research in the gift of the 

 Royal Society of London. For clearness and convenience of reference 

 the subject will be treated under the following heads : — 



I. Introductory remarks indicating the aims of the investigation. 



II. Colonsay and Oronsay — description of the preglacial platform. 



III. Islay. 



IV. Mull and Iona. 



V. The Treshnish Islands. 

 VI. Conclusion, setting forth the present state of the subject. 



I. Introduction. 

 In the isostatic problems connected with the Ice Age the deter- 

 mination of the preglacial sea-level is of primary importance. It is 

 a datum to which subsequent oscillations of the shoreline may be 

 referred, and as such affords a criterion for the discrimination of 

 temporary and permanent deformations of the crust. A permanent 

 glacial or postglacial deformation must show itself as a difference in 

 level between the preglacial and present shorelines. On the other 

 hand, a temporary oscillation, if complete, leaves no record of this 

 kind, although it may be readily demonstrable in other ways. Now 

 in the British Isles there is abundant evidence of both glacial and 

 postglacial oscillations, and it is- obviously important to determine to 

 what extent the displacements effected have been temporary or 

 permanent. The tracing of the preglacial shoreline has therefore in 

 this respect a very special interest, and if successfully carried out 

 might ultimately lead to the establishment or rejection of Jamieson's 

 isostatic theory of the quaternaiy oscillations of sea-level. 3 



1 Interglacialists will excuse the use of this term in the sense of prior to the 

 only apparent general glaciation of the district in question. 



2 T. F. Jamieson, " On the History of the Last- Geological Changes in 

 Scotland " : Quart. Journ. Geol. Soc, vol. xxi, p. 178, 1865. 



DECADE V. — VOL. VIH. — NO. HI. 7 



