NATURAL HISTORY SOCIETY OF GLASGOW. 155 



terraces tlirougli openings, and strew the face of the eastern front 

 of the island. All these stones are rough, sharp-edged, and belong- 

 ing to the syenite of the Craig. In the deposit of boulder clay, I 

 found only two pieces of this syenite, all the rest being specimens 

 of other formations — some of pure quartz, others probably meta- 

 phorised sandstones, while a few of them are derived from shale, 

 as they have the distinctive clay nodulite character of that 

 formation. 



These facts, in brief, are what I observed during my stay on the 

 Craig, and from these, I think, we can justly draw some deductions 

 Avhich, when viewed with those theories which are generally re- 

 ceived, will add some Httle interest to the solitary rock of 

 Ailsa. 



As you are aware, the relative positions of land and water have 

 changed considerably during a comparative short period of geologic 

 time. The Craig, like unto some monolith, marks in some degree 

 those changes which I will try briefly to lay before you. Without 

 any great stretch of imagination, we must suppose the Firth of 

 Clyde to be filled with successive stratas of silurian, old red, and 

 carboniferous ages ; then from Ayrshire coast to Kintyre, the strata 

 developed on either side will be continuous. From out these 

 strata the peaks of Arran, and the silurian lulls of the Southern 

 Highlands will rise, and the centre, but as a boss of rock, Ailsa 

 will be noticed. The formations in those parts, left exposed, fully 

 bear out this arrangement, for in Arran the shale band is esti- 

 mated to be 2000 to 3000 feet thick, while in that island, in Bute, 

 Cumbraes, and along the coast, the old red, and the carboniferous 

 are developed to considerable thickness. On the opposite coast of 

 Kintyre, Mr Thomson has found the carboniferous strata. That 

 these strata once filled Avhat is now occupied by the waters of the 

 Firth, we will take for granted, but a time came when the sub- 

 sidence of the land took place; then began the action of those 

 abrading agencies of water and ice, which gradually wore away the 

 strata, and carried it out to be deposited in the " silences of the 

 central sea." The powerful currents oj)erating from the north- 

 west tore against the peaks of what we call Arran, and, sweeping 

 round, were carried in full force upon the intermediate land, and 

 probably, aided by the uptilting of the strata, slowly but surely 

 revealed the bluff, bare front of Ailsa. Successive years rolled on, 

 and still the action of waves, aided by glaciers from the Highlands, 



