EXCURSION XI. 153 



The front of Windmill Hill is formed, through about half 

 its height, of columnar felspar porphyry, similar to that of 

 Dun-Dhu, already described (Art. 41). The columns are 

 four, five, and six-sided, with flat joinings; the pillars lean 

 in various directions. The junction with the sandstone 

 below is nowhere visible. About the middle of the front of 

 the hill a mass of altered sandstone, 12 feet wide, is imbedded 

 in the porphyry. Close to this is a whin dike, running 

 N. 10 E. Several others traverse the ridge of porphyry 

 another example of the posteriority of the common trap or 

 whinstone to all the rocks of Arran. The plane of contact 

 between the sandstone and porphyry gradually ascends west- 

 ward, and, on reaching the S.W. shoulder of the hill, we 

 find a wedge-shaped mass of sandstone connected with the 

 body of this rock below, and apparently separating the 

 porphyry from the granite which immediately succeeds. No 

 true contact of these rocks is observable. Sandstone, slaty 

 pieces of porphyry, granite sand, and bits of granite are seen 

 lying about, mixed up confusedly, and no line of demarcation 

 can be laid down. Presently the hill-side shews granite 

 only. Eastwards all the high ridge of Windmill Hill is 

 porphyry; sandstone is seen at the north base, but no contact 

 is visible. The base of the porphyry is an intimate mixture 

 of felspar and quartz, of a gray or bluish-gray colour, with 

 imbedded felspar crystals, bits of quartz, and occasionally well- 

 formed crystals of this substance. We have already (Art. 11) 

 described at sufficient length the relations of the sandstone to 

 the Ploverfield granite, to this porphyry, and the syenite of 

 the hill-sides westwards, into which the granite seems to 

 shade off, and need not now recapitulate. The series forms 

 an interesting study. The granite, porphyry, and syenite 

 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. The occurrence in this sharp-crested 



