154 GEOLOGY OF ARRAN. 



hill of an erupted rock like porphyry must be explained as we 

 explain the existence of granite in mountain peaks, or basaltic 

 lavas in the mural precipices of Mull and the Giant's Causeway. 

 72. Passing round the heads of Glen Dhu and Glen Cloy 

 (Art. 11), we come over the edge of high sandstone cliffs 

 bounding Glen Cloy on the south. On a grassy ledge under 

 the western part of these cliffs there occur two remarkable 

 dikes, producing a highly interesting change upon the sand- 

 stone. A large body of this rock, between the two dikes, is 

 altered to the state almost of quartz rock, and beautiful 

 crystals of amethyst are developed in it; quartz crystals, 

 both colourless and with a slight tinge of yellow, 

 also occur. The dike on the S.E. side is a brick-red 

 porphyry, resembling that on the Corriegills shore ; it appears 

 also in the cliffs above, where, by wearing, a fissure is 

 formed upon it; the dike on the N.W. side is of greenstone. 

 There is a third dike or mass of trap outside the porphyi-y; 

 but neither its relation to the others, nor of these to another, 

 can be well made out. The locality altogether is fully as 

 interesting, on account of the striking metamorphism, as any 

 in the island. The isolated high " fairy knolls," called the 

 Sheeans, over the head of Glen Cloy, are trap. Passing 

 from them to the head of Lamlash, or Ben-leister Glen, we 

 meet with a few low knolls of like composition, but no body 

 of overlying trap. Lamlash Glen affords a fine section of 

 the carboniferous series : red limestone and red marl at the 

 top of the glen, and lower down two other considerable beds 

 of limestone, the last being about a mile and a-half from 

 Lamlash. They contain the usual fossils, and occur amid 

 massive sandstones. A short distance below the lowest 

 limestone a dike of felspar rock, or quartziferous porphyry, 

 fifteen to twenty feet wide, crosses the bed of the river, 

 ranging nearly N. and S. The sandstone is greatly altered 

 by it. Between this point and the alluvial plain there are 

 several veins or beds of claystone and greenstone, breaking 

 through the sandstone of the river bed. 



