262 Dr. Mac Culloch ow the 



Few marks of sucli a large laminar disposition are to be ob- 

 served in the syenites, clay-stones, and clink-stones, or the por- 

 phyries derived from these two latter rocks; alfhongh the Isle of 

 Mull presents some remarkable exceptions. No relation can 

 therefore be inferred betwreen the direction of the exfoliation and 

 that of a larger mass ; and accordingly we can only conclude 

 that, whatever surface is thus found exfoliating, the direction 

 of this change relates to that of the exposed surface. Yet, as 

 it will, I believe, always be observed, that none of these rocks 

 exfoliate in two directions at any one place, it is probable that 

 this tendency does actually bear some relation to the internal 

 structure of a mass, no other indications of which are visible. 



The number of successive laminae which may be detached 

 from any of the rocks last named, is various; but I know not 

 that it bears any constant relation to the species. I have only 

 observed, that when they do occur, they are more numerous in 

 the softer clay-stones than in the syenites, or rocks with a base 

 of clink-stone. If one, two, or three, can be detached in suc- 

 cession, that facility soon ceases; and, after some partial indi- 

 catures of future desquamation, the rock is found to be massive, 

 and to give no marks of the future renewal of a similar process, 

 although there can be little doubt that it will continue in suc- 

 cession as the preceding laminaj become detached. Such 

 laminos generally present the same average but variable thick- 

 ness which has already been mentioned. The surfaces are also 

 occasionally undulated or irregular, and, in a few instances, 

 slightly curved. From preserving the tenacity of stone, it is 

 generally supposed that they are unchanged, and present the 

 natural characters of the rock to which they belong. But this 

 is not the fact; and they will be found, in this important cir- 

 cumstance, to differ essentially from those laminae already 

 described which pervade the interior of veins, and which are 

 conceived to arise from an internal concretionary structure. 

 The deceptioit has in these cases arisen from the great depth to 

 which the rocks of this family sometimes weather without losing 

 their tenacity, undergoing little apparent change but that of 

 colour. Hence the solid parts of such rocks, when broken even 



