178 GEOLOGY OF ARRAN. 



bands ; but the porphyry, as a mass, may be said to prevail 

 upwards, the two other rocks being in greater proportion 

 below, except that on the east side the porphyry comes down 

 to the level of the beach. The entire mass of the headland 

 is wedge-shaped, narrowing downwards; the tipper edge of 

 the porphyry cliff being on the same level on either side as 

 the strata of sandstone, the ends of which abut against the 

 trap rock throughout. Though the sides converge rapidly, 

 the termination of this wedge-shaped mass is not seen, the 

 igneous rocks running out under low water and cutting off 

 the sandstone. Dikes of common trap intersect the whole 

 series from top to bottom of the cliffs ; and a little way west 

 of the headland a dike of quartziferous porphyry, very 

 distinct from the porphyry of the headland, intersects the 

 sandstone 011 the beach. 



The Black Cave in the front of the headland has been 

 excavated in a vein of rotten trap, between two basaltic 

 walls, and extends about forty paces by fourteen, with a 

 height of seventy or eighty feet. Rocks have fallen from the 

 roof making an opening overhead. The vista across the sea 

 is finely closed by the erupted cone of Ailsa. Loitering here 

 may turn out to be very embarrassing, as the front of the 

 cliff is not passable at or near high water; and at certain 

 times of tide we may get into such an awkward situation 

 that our only chance of escape is " to swim for it." 



The cliffs westwards present for some distance porphyry 

 and greenstone, apparently in alternating bands; but it is 

 difficult to make out the exact relations. Thence to the 

 mouth of Torlin Water, the shore and cliffs are occupied by 

 variously coloured sandstones and marls, with occasional 

 nodules of limestone and ironstone. The platform of the 

 beach is crossed by multitudes of dikes, mostly of basalt and 

 greenstone, which usually produce a decided alteration on 

 the sandstone strata. The road from Benan to Lag, which 

 runs a little way inland, shews generally, along the undulat- 

 ing grounds on either side, beds of claystone beneath the 



