THE NATURAL HISTORY OF AILSA CRAIG. 1 39 



or less perpendicular, and where it is seen the columnar structure 

 disappears and the rock is much finer grained, the blocks which 

 have fallen from the cliffs of this structure on the north-west side 

 being the largest on the shore. It is from this part that the 

 curling-stone maker selects his material, which he calls the " blue- 

 hone " and the " red-hone," although the blue is not very blue, nor 

 the red one very red. There are other two varieties called " pepper 

 and salt" and "Ballochmyle." There is, however, a pretty red 

 variety of the syenite to be obtained, and it appears to be a result of 

 what I might call extra-contact metamorphism. This will require 

 a short explanation. Passing through the syenite in an N.N.W. 

 and S.S.E. direction there are a number of respectable, but not 

 great-sized, dolerite dykes, some of them at parts amygdaloidal, and 

 it looks as if the structure described above with its N.N.W. and 

 S.S.E. trend (which may be owing to crystalline orientation, or 

 perhaps to pressure as I suggested in 1893 when I wrote a short 

 description of the island — it may of course be rhyolitic) may have 

 had something to do in determining the trend of the dykes. One 

 of these dykes occurs about 100 yards N.E. of the south horn 

 building. For eight feet from the side of the dyke the syenite is 

 of the usual greyish, green-tinted variety (a shade greener than 

 usual, at least close on the dyke), then there are three feet of the 

 reddest stone on the island shading away outwards for a few feet 

 until the usual colour of the rock at this part is reached. The 

 dykes are all much of the same quality of dolerite, not a very fine 

 grained variety except where in contact with the syenite, where 

 the dolerite is often much jointed. They are mostly prismatic 

 across, but the prisms are not at all clean-cut. In some of the 

 dykes there are vertical lines which divide them into several parts, 

 generally three (and the line of a dyke is often seen to " gink"). 

 This is probably the result of cooling, at least there is no apparent 

 change in the material to show that there had been several 

 injections. The dykes closely resemble in mineral structure many 

 of those which cut through various strata in the west of Scotland, 

 and are probably of Tertiary age. As to the age of the syenite, 

 it must of course be older than the dykes, and younger than the 

 rock from which it rises — which I have already shown is probably 

 old red sandstone. I am not aware that the drift period is 

 materially represented on Ailsa, at least I saw no boulder-clay, 



