143 



are all posterior to the sandstone ; but we have as yet no means of 

 knowing the age of the porphyry in relation to the granite. The 

 common greenstone of the dikes, and detached knolls upon the 

 plateau southwards, is the newest of all these igneous products. 

 Sir C. Lyell, following Prof. Ramsay, remarks on "the overlying 

 trap ceasing here very abruptly on approaching the boundary of the 

 great hypogene region, and terminating in a steep escarpment facing 

 towards it as at 5 /, fig. 702, p. 591. When in its original fluid 

 state it could not have come thus suddenly to an end, but 

 must have filled up the hollow now separating it from the hypo- 

 gene rocks, had such a hollow then existed" (El., edit. 1855, 

 p. 592). By this "overlying trap" is meant the common trap of 

 the southern plateau, as is seen by referring to Ramsay's section, 

 copied by Lyell; but there is no such rock here. The porphyry is 

 the only erupted rock, and its occurrence in this sharp-crested hill 

 must be explained as we explain the existence of granite in moun- 

 tain peaks, or basaltic lavas in the mural precipices of Mull, and the 

 Giant's Causeway. There is no igneous rock of any kind " at the 

 points 5 f t " overlapping the carboniferous sandstones and resting on 

 the old red sandstone (see Art. 46, sub. Jin.) 



99. Passing round the heads of Glen Dhu and Glen Cloy (Art. 50), 

 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 sandstone. 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 green- 

 stone. There is a third dike or mass of trap outside the porphyry; 

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

 well made out. The locality altogether is fully as interesting as 

 any in the island. The isolated high round knolls, called the Sheeans, 

 over the head of Glen Cloy, are trap. Passing from them to the 

 head of Lamlash or Bein-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 



