50 GEOLOGY OF THE LOTHIANS. 



loses its compact for a granular structure, and its grey co- 

 lour for one more or less white, these marks of alteration 

 becoming gradually less conspicuous as the distance from 

 the igneous mass increases. The various individuals which 

 comprise the trap series, evince, by two modes of position, 

 a formation posterior to that of the Neptunian strata, as 

 well as to those older igneous masses with which they may 

 happen to be connected. The first is that of the vein or dyke ; 

 the second is that of the overlying mass. In the vein, a body 

 of trap of varying magnitude crosses the strata at an angle, 

 or, originating in a mass which does, obtrudes itself be- 

 tween the strata parallel to their direction. In the Lothi- 

 ans the greater number of veins traverse the strata in this 

 conformable manner, and the reason of this may in all pro- 

 bability be found in the fact of the planes of stratification 

 being those which would afford the greatest facility of se- 

 paration. Trap, in the overlying position, forms the most 

 of the great trap deposits of the Lothians ; when it occurs 

 as veinous masses it is seen in its progress through the stra- 

 ta ; but when as overlying it is to be considered as more or 

 less in the position which it assumed when, after rising 

 through the previously formed rocks, it flowed over their 

 surface. The changes in position which the various stratified 

 rocks exhibit are very great ; and all are exactly such as 

 might be conceived to attend the violent expulsion of a 

 more or less fluid body from the interior. Fragments of 

 the disturbed rocks are enveloped in the erupted mass, and 

 vary in size from a few inches to many yards. Contor- 

 tion, a striking position of strata, and one which is most 

 indicative of their having been subjected to violent action, is 

 in the three Lothians frequently well exhibited. In consider- 

 ing in detail the relations of the unstratified rocks of the Lo- 

 thians to the stratified, we shall not endeavour to follow any 



