INTRODUCTION TO THE TRAP ISLANDS. 235 



INTRODUCTORY REMARKS ON THE TRAP ISLANDS/ 



IN giving to this group the name of the Trap islands 

 I have been guided by the nature of the predominant 

 rock ; although it will appear when the individuals 

 are described, that some of them, as Pabba for example, 

 contain no masses of that substance. But since, together 

 with trap, some of these islands present conspicuous 

 and interesting tracts both of the primary and secondary 

 rocks, the illustration of which depends in a great mea- 

 sure on the structure of all the neighbouring parts 

 jointly considered ; the few here included, in which that 

 rock is not present, could not have been omitted without 

 inconvenience : the deficiency of arrangement is fully com- 

 pensated by the advantage gained in point of illustration. 



Before proceeding to consider these islands in detail, 

 it will not be unprofitable to take a general view of the 

 relations which they bear to each other, and to the con- 

 tinent near which they lie. To aid this view the general 

 Map may be consulted. 



The trap is obviously divided into two principal groups, 

 Sky being the centre of the one and Mull of the other; 

 these being however connected by detached masses occu- 

 pying either the intermediate islands or parts of the con- 

 tinent. The Shiant isles and the remarkable rocks at Loch 

 Maddy formerly described, are found at the outskirts 

 of this boundary, from which however St. Kilda may 

 be excluded, on account of its total want of probable 

 connexion. 



The first feature which arrests the attention in con- 

 sidering these islands, is the nonconformity of outline 



* See the table at the commencement of this work, and also the 

 general Map. 



