350 OTHER CLYDE ISLANDS. 



the same age as that of S. Bute, most probably lower car- 

 boniferous. The only varieties of trap occurring here are 

 coarse and fine greenstone, and basalt ; the fine greenstone 

 has often a porphyritic structure, from the imbedded felspar 

 crystals, thus forming the variety incorrectly termed trap 

 porphyry. The chief interest of the island is in its whin 

 dikes. These alter the strata remarkably : the sandstone 

 has been fused by them, and reconsolidated into a substance 

 closely resembling a dark quartz rock; simple induration is 

 induced at a greater distance from the dike. Many of the 

 dikes stand out boldly from the adjoining sandstone, which 

 has been worn away on either side, the amount of wearing 

 in the ancient tideway, compared with that in the present, 

 affording a rude measure of the time during which the sea 

 remained at the higher level. Attention was first called to 

 these remarkable dikes by Mr. Smith of Jordanhill. The 

 best example is seen a little to the east of the entrance of 

 Millport harbour, where a large dike, rising from the sea 

 level like a huge wall, runs far up along the hill-side, raised 

 as if by art above the surface of the fields. On the shore of 

 the mainland opposite, another dike, having the same 

 direction and apparently a prolongation of this, stands out 

 in the same manner from the surface of the sandstone. The 

 similarity of the two masses of rock and their former con- 

 nection are pointed at in the legend that this strait was once 

 spanned by an enormous bridge, raised by the hands of mighty 

 wizards, of which the only portions that remain are these two 

 ancient abutments.* 



Such projecting masses of trap are not confined, however, 

 to the shore; they occur in the inland parts, where they 

 are, of course, not so easily accounted for. We can hardly 

 suppose them due to atmospheric causes, as there are no 

 streams to carry off such worn materials ; and they are most 



Quart. Jour. Agric., and Trans. High. Soc., No. xliiL; or voL ix. 

 p. 430. 



