194 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASOOW. 



bank of the Doon, crossing that river near its month, and thence 

 along the sands towards the Heads of Ayr. At low water, 

 along this stretch of shore, the cement - stone division of the 

 calciferous sandstones, which form the lowest story of the 

 carboniferous formation, may be seen cropping out from under 

 the sands, and dipping gently towards the north-east, so that 

 lower and lower members of the series, to a thickness of many 

 hundred feet, are crossed as we approach the " Heads." The 

 first object of particular interest is the ruine<l fortalice of 

 Greenan, jjerched on the edge of a tall escarpment of volcanic 

 tuff, which in places appears to traverse, and in places to be 

 interstratified witli, the cement stones. Embedded in the tuff 

 are numerous fragments, lx)th of volcanic and of sedimentary 

 rocks, which were ejected as bombs from the crater of some 

 adjacent contemporary volcano, belonging to the same period of 

 volcanic activity as that to which the bedded traps of the 

 Campsie, Kilpatrick, and Eenfrewshire hills are due. A sliort 

 distance beyond Greenan a dolerite dyke, 25 to 30 feet thick, 

 and probably of Tertiary age, crosses the shore. This dyke has 

 brought up and enclosed within itself a mass of fine-grainetl 

 shale, which is prolmbly composed, like the famed Water of Ayr 

 stones, of finely comminuted volcanic ash. The metamorphic 

 action of the dyke has induced a beautiful spherulitic structure, 

 in many parts of the shale of which some good specimens were 

 obtained. 



A mile beyond the dyke we come to the most northerly of 

 the " Heads " of Ayr, a great mass of volcanic agglomerate, 

 which has been scarped into tall sea cliffs and weathered into 

 fantastic forms by the combined action of marine and aerial 

 denudation, thus exposing the core of what must have been a 

 great volcano, and probably that to which the tuffs of Greenan 

 are due. On the shore in front of the cliffs the cement stones 

 are found to be greatly disturbed and indurated by volcanic 

 intrusions, while all round the " Head " the cement stones are 

 seen to dip inward towards the agglomerate — a characteristic 

 feature of stratifietl rocks in contact with a volcanic rent, which 

 appears to be due to the contraction of the core in cooling, 

 drawing the truncated edges of the strata downwards towards itself. 



Beyond tlie ' Head " is another stretch of sandy shore, in 



