462 DB. A. J0WETT OX THE [Oct. I913, 



of fine sediment, probably in water of no great depth, as the 

 sediments are generally current-bedded. 



Near the fault mentioned above (p„ 461), a highly-contorted layer 

 of sediment, coarser than usual, was found interbedded with the 

 lava. This is composed almost entirely of volcanic debris, and is 

 probably one of the finer-grained beds associated with the volcanic 

 conglomerates. 



South of the fault, the felspar-phenocrysts so characteristic of 

 the lavas met with on the north and west are no longer found — the 

 new rocks being generally fine-grained, with obvious red phenocrysts 

 representing pseudomorphs after olivine. These rocks are olivine- 

 b as alts. This type of lava extends southwards continuously for 

 a mile and a quarter, as far as Fishtown of Usan. 



Up to this point the rocks are exposed only between the tidal 

 limits, the low cliffs being here grassed over. Excellent horizontal 

 sections, however, are exposed ; and, where the harder rocks have 

 resisted the action of the waves, crags 10 or 12 feet high provide 

 good vertical sections. A few faults are traceable, but I could find 

 no evidence that their throw is considerable. 



The upper portions of the lavas are amygdaloid al, much fissured, 

 and covered by a variable thickness of slaggy fragments, so that, 

 with the sediment deposited in the crevices, they resemble con- 

 glomerates in appearance. In some cases, the rock- fragments are 

 rounded, forming true conglomerates, often of no great thickness, 

 intercalated in the series. Sometimes the compact base of a lava- 

 stream rests directly upon amygdaloidal or even compact lava with 

 sandstone-filled fissures, without any intervening ' conglomerate,' 

 probably indicating that the loose material usually present has 

 been removed and deposited a little distance away as a true 

 conglomerate. 



The frequent redness of such slaggy and conglomeratic beds has 

 been attributed by Sir Archibald Geikie 1 to oxidation during the 

 lapse of time between two successive outflows of lava. 



The compact lavas vary much in colour, the fresh rocks being 

 almost black, but passing, with varying degrees of weathering, 

 through shades of grey, purple, brown, and red to a lilac tint." 



A quarter of a mile south-east of ' Mains of Usan,' much- 

 weathered lavas are clearly overlain by a massive conglomerate 

 full of well-rounded boulders measuring up to 3 feet in diameter. 

 At the base of the conglomerate occurs a non-persistent layer of 

 red sandstone, sometimes 18 inches thick. 



The matrix of the conglomerate consists of a coarse purplish 

 sediment, which, when microscopically examined, is found to 

 be composed almost entirely of small fragments of different kinds 

 of volcanic rock. The" boulders in the conglomerate, though all of 

 volcanic origin, are not all of the same type, some exhibiting 

 porphyritic felspars. 



At the surface of the conglomerate is a layer of 12 to 18 inches 



1 ' Ancient Volcanoes of Great Britain ' vol. i (1897) p. 301. 



