1871.] GEIKIE — TERTIiET TOLCAWTC ROCKS. 285 



more than 1000 feet. Some of the noblest hills of the Inner 

 Hebrides are but solitary outliers left standing amid the ruin of the 

 great sheets of solid rock of which they once formed a part. Ben 

 More, in Mull, though more than 3000 feet high, is only a mag- 

 nificent fragment of the huge pile of volcanic material which for- 

 merly swept over what are now the deep glens and fjords of Mull. 

 The long lines of imposing cliff with which the basalt plateaux front 

 the Atlantic all through these islands, from the Fair Head of Antrim 

 to the far headlands of Skye, tell everywhere the same tale of vast 

 and continuous denudation. Great, therefore, as the area is over 

 which these rocks are now to be traced, it covers but a small part of 

 its original extent. 



These prefatory remarks may suffice to show the general nature 

 of the subject of which I propose to treat, and I shall now proceed 

 to describe in some detail a district in which some of the phenomena 

 are typically displayed. The area which I have selected for this 

 purpose is the island of Eigg, partly on account of its simplicity of 

 structure, and partly because it presents to us a more striking picture 

 of the vast duration of the Tertiary volcanic period in Britain than 

 any other space of like size with which I am acquainted. My 

 observations are the result of a survey made by me of the island in 

 the year 1864. In this excursion I was accompanied by my friend 

 and former colleague Professor Young, of the University of Glasgow, 

 who devoted himself to the palaeontology of the island. It was our 

 original intention to combine our observations in a joint memoir. 

 Circumstances having occurred, however, to delay the proper exami- 

 nation of the fossils, it has been judged expedient to publish, in the 

 mean time, my own observations on the volcanic geology of the 

 island, leaving the oolitic strata and their fossils to form the subject 

 of a future communication. 



THE ISLAND OF ETGG*. 



A. Physical Features and Geological Strtjcture. 



In the chain of the Inner Hebrides, broken as it is in outline and 

 varied in its types of scenery, there is no object more striking than 

 this island. Though only about five miles long and from a mile and 

 a half to three miles and a half broad, and nowhere reaching a 

 height of so much as 1300 feet, this little island, from the singu- 

 larity of one feature of its surface, forms a conspicuous and familiar 

 landmark. Viewed in the simplest way, Eigg may be regarded as 

 consisting of an isolated part of one of the great basaltic plateaux 

 which, instead of forming a rolling tableland or a chain of hiUs with 

 terraced sides, as in Antrim, Mull, and Skye, has been so tilted that, 

 while it caps a lofty cliif about 1000 feet above the waves at the 



* The spelling of the G-aelic names on the map and in this memoir has been 

 kindly revised for me by my friend Mr. Alexander Nicolson, advocate, whose 

 name will be a sufficient guarantee for their accuracy. 



VOL. XXVII. — part I. X 



