138 THE NATURAL HISTORY OF AILSA CRAIG. 



facts of the case, but a sill or lacolite at least 1100 feet thick is 

 apt to stagger the most blase geologist, but, of course, it may not 

 all have taken place at one injection, and I shall now proceed to 

 consider the facts in support of this theory. There are the 

 columns which form a strong buttress to the theory, and a large 

 part of the northern half of the west coast shows very decided 

 lines, dipping gently to the north, which may be partings between 

 small sills injected at different times, or they may represent the 

 successive cooling of larger sills, so that the sill or lacolite theory, 

 is the one most strongly supported by the geological features of 

 the island. I may here remark that there is a considerable 

 resemblance between this structure of the Ailsa syenite and the 

 slabby structure so well seen in many parts of the Arran granite. 

 Ailsa rock or syenite exhibits a variety of structures and colours 

 or shades. Thin plates of it prepared for the microscope show 

 that it contains all the constituents of a typical granite, and it has 

 in consequence been called micro-granite. It contains a rare 

 mineral (abundant here) once thought to be a variety of hornblende. 

 This mineral has been called riebeckite, and is in largest crystals 

 — up to a quarter of an inch long, and of a dark-green colour — 

 where the rock assumes its finest columnar form. The columns 

 of syenite about the middle part of the west coast show what 

 appears to be a remarkable style of reining or jointing ; this 

 cannot be examined closely, but from the columns being very 

 irregularly divided into "compartments" it gives to them a strange 

 appearance, and one I have not seen elsewhere. These columns 

 may have been weathering since raised-beach times. Besides the 

 magnificent columnar structure so well seen at the south end and 

 on the west side, the columns being of considerable width and 

 reaching to perhaps 300 feet high, there is a platty structure well 

 displayed in a small area on the south-west coast, the plates dipping 

 gently to the water. A similar structure (but much weathered) 

 is also seen near the summit of the island, and here it is not far 

 from being horizontal. The sill or slab-like structure of the north- 

 middle part of the west cliffs has already been referred to. At the 

 north-west, north, and north-east sides weathering has also brought 

 out a structure which when viewed from a short distance gives to 

 the syenite a stratified-looking appearance. This structure trends 

 in a north-north-west and south-south-east direction, and is more 



