236 INTRODUCTION TO THE TRAP ISLANDS. 



they present to the general north-easterly bearings oi 

 the coasts, which have already been stated in a cursory 

 way to depend on the direction of the strata, and will 

 hereafter be more fully seen to proceed from that cir- 

 cumstance. This contrast is peculiarly remarkable in 

 Sky, where the larger part of the island presents a ten- 

 dency different from that of the smaller portion. The 

 former consists of trap ; whereas the latter is formed of 

 the stratified rocks both primary and secondary, and 

 maintains its general position with respect to the con- 

 tinent; preserving, as will hereafter be shown, the regular 

 directions of those rocks where they occur in more 

 continuous masses. 



The nonconformity of the outline of Mull is less 

 apparent; yet its longest shore, which together with 

 the opposite land of Morven includes the Sound, will 

 also be seen to form a large angle with the predominant 

 lines of the western coast. 



On the same map the extent of the secondary strata 

 may be traced, and it is apparent that there is a very 

 general coincidence between the two classes of rock, or 

 that the trap is confined, with but slight exceptions, within 

 the same limits as the secondary region. The only portion 

 at much variance with that rule is the porphyritic district 

 of Glenco, on the mainland ; but of the real connexion of 

 this with the Trap islands there are considerable doubts, 

 which I shall have occasion to state hereafter. 



Our present knowledge of the trap rocks is perhaps 

 not yet sufficiently extensive to allow us to speculate 

 on this connexion between them and the secondary 

 strata. We have no reasons a priori to expect that 

 they should be found in this situation exclusively, nor 

 is the rule indeed by any means universal; yet it is 

 remarkable that in Scotland as well as in the neighbouring 

 country, Ireland, almost all the great overlying districts of 

 trap are principally connected with Iht-ho strata. If we con- 

 sider the probable causes in which they have originated, 

 a 



