ON THE GEOLOGY OF ARDROSSAN. 605 



angular and subangular fragments of a holocrystalline igneous rock which, from their 

 rusty brown or greenish colour, are particularly conspicuous upon the wave-swept surface 

 of the Inches lava. These under the microscope are found to be fragments of dunite 

 and augite-peridotite, composed entirely of rounded grains of olivine with occasional 

 irregular interstitial crystals of augite and pictotite. # They appear to have suffered 

 little or no corrosion by the basic magma. That they are not, however, of the nature 

 of olivine nodules but are real xenoliths, is evident both from their general angular shape 

 and from the occasional presence of a zone of brown glass surrounding them. Xenoliths 

 of granite and of quartz-orthoclase pegmatite are also common, and some slight corrosive 

 action has occasionally taken place around these especially in the neighbourhood of the 

 quartz. The most interesting and intense action of the basic magma is, however, to be 

 observed round isolated xenocrysts of quartz and felspar, which sometimes reach a 

 considerable size. The felspar xenocrysts belong commonly to acid oligoclase, and have 

 suffered the usual regular penetration by the basic magma along the cracks and cleavages. 

 The quartz xenocrysts, if small, are surrounded by a ring of tiny augite crystals, or by 

 an outer zone of augites and an inner of brown glass. If the quartz crystals are large, 

 however, and if they have suffered much corrosion, the early formed outer ring of 

 augites may again be absorbed and in the broad zone of glass there may develop 

 skeletal isotropic microlites of felspar, which later become radiating groups of well- 

 defined and often repeatedly twinned crystals of acid plagioclase, most commonly of 

 oligoclase.t (See PI. II., fig. 3.) 



(b) The Carboniferous Intrusive Mocks. 



1. The Castle Craigs Picrite. 



This rock, which may be termed picrite from its most interesting modification, m 

 reality passes regularly upwards from picrite to hornblende dolerite along the whole 

 length of the sill (fig. 1). 



The lowest layer (1) in contact with a white sandstone is a fairly coarse-grained 

 greenish rock, flecked with pink and white. Above this is a coarse-grained dark-green 

 rock (2), in places rendered friable through exposure, which makes up more than half 

 of the intrusion. Traced upwards, this rock becomes somewhat finer grained ; spots 

 and streaks of pink felspar begin to appear in it, and these become more and more 

 abundant until the rock itself assumes a pinkish colour (3). At the same time coarse 

 red felspathic knots become frequent, the usual central cavity being filled with calcite 

 or analcime. The upper portion of the sill (4) is very fine-grained and pseudo- 

 laminated, showing an alternation of narrow reddish and greenish bands parallel with 

 the upper surface of the sill and crossed occasionally by tiny pink felspathic veins. 

 This fluidal banding becomes more and more pronounced as the margin is approached. 



* Cf. " The Tertiary Igneous Kocks of Skye," Mem. Geol. Sur., p. 69. 



t Cf. Tertiary Igneous Rocks of Skye, pp. 361, 481 ; Geology of North Arran, pp. 114, 116. 



