40 Lord Greenock on the Igneous Rocks 



trated by the experiments of the late Sir James Hall ; or by 

 their having been originally formed as dykes, at considerable 

 depths, either below or among the strata. 



The most striking of the phenomena which are to be seen in 

 many of the hills near Edinburgh, are the apparent interstratifi- 

 cation of the trap and secondary rocks, owing, in some cases, to 

 the igneous matter having carried up portions of the latter, which 

 are, in this manner, placed above the former on their summits, 

 and in others to its having burst through, and overflowed the 

 surface of the strata, or been injected between them ; while, at 

 the same time, it was probably lifting them up from below. Frag- 

 ments broken away from the stratified rocks, by the passage 

 through them of the igneous fluid, are also frequently seen im- 

 bedded in the trap, and the former are very generally observed 

 to be altered in their nature and appearance, when in contact 

 with the latter. Different varieties of trap-rocks are often met 

 with in the same hills, shewing that they must have continued to 

 be erupted at different intervals, and under varying circum- 

 stances, during a long period ; but, in the absence of every for- 

 mation superior to the coal measures, there are no data whatever 

 by which their relative ages may be determined. 



The igneous rocks almost invariably conform to the same ge- 

 neral dip as the stratified rocks with which they are associated, 

 thus affording additional evidence of their having been up- 

 lifted together by some common cause. A very distinguished na- 

 turalist has pointed out to the author the important fact, that 

 the trap-hills appear in general to surround basins, often of con- 

 siderable extent, in which the rocks of both descriptions are seen 

 to dip outwards from a common centre, forming, as it were, val- 

 leys of elevation on an immense scale, and that the environs of 

 Edinburgh afford an example of one of these basins or valleys ; 

 for, if lines be drawn north and south from Burntisland to the 

 Pentland Hills, and east and west from Salisbury Craigs to Cor- 

 storphine Hill, they will form the anticlinal lines (or nearly so) 



