Vol. 52.] BAS ALT-PLATEAUX OF NORTn-AYESIERN EUROPE. 379 



Sandstone. This deposit has been correctly identified by Prof. Judd 

 ■with the similar strata which in Skye and elsewhere on the west 

 coast of Scotland underlie the Liassic series. It is here about 

 10 or 12 feet thick, reddish and yellowish in colour, and distinctly 

 calcareous. Its component pebbles consist largely of Cambrian 

 (Durness) limestone, quartzite, and Torridon Sandstone — rocks which 

 all occur in situ in Sleat. It may be compared with the limestone- 

 conglomerates of Strath, and those which underlie the Lias at Heast 

 on Loch Eishort. 1 That here, as elsewhere in this region, the 

 basement-conglomerate was followed by the rest of the Lias and 

 Oolites may be inferred with some confidence from the copious 

 development of the Jurassic Series a few miles off, both to north and 

 south. But the whole of this overlying succession of formations 

 has here been swept away, and, but for the protection afforded 

 "by the eruptive rocks of Rudh' an Iasgaich, the conglomerate would 

 likewise have disappeared. 



Above the conglomeratic band lies a sheet of intrusive rock, which 

 in one place has apparently cut it out, so as to rest directly upon 

 the Torridon Sandstone (a in fig. 23, p. 380). The decay of the softer 

 detrital rock underneath has caused the sill to break off in slices, 

 which have left behind them a bold mural escarpment. 



The rock of this sill (b b) is a rather coarsely-crystalline porphyritic 

 olivine-dolerite, which towards the north attains a thickness of 

 about 70 feet. It exhibits the usual prismatic jointing, though less 

 perfectly than some of the Trotternish sills already referred to. 

 Besides these vertical joints, it is also traversed by a system of 

 horizontal divisional planes which, though somewhat irregular in 

 their course, run, in a general sense, parallel to the upper and 

 under surfaces of the sill. 



It seems to have been along this transverse series of joints that 

 a second sill (c), 5 or 6 feet thick, has been injected. The material 

 of this younger intrusion is a black, finely crystalline dolerite or 

 basalt, with rudely prismatic jointing. Its most striking feature, 

 besides its regularity of position and persistency for several hundred 

 yards as a platform along the shore, is the basalt-glass which marks 

 both its under and upper surfaces of contact, and which is here 

 developed upon a scale the equal of which I have not met with 

 among the Tertiary sills of this country. 



The selvage of glass appears as a black tar-like layer, varying 

 from a mere film to 2 or 3 inches in thickness. It is found not 

 only on the upper and under surfaces, but descends along abrupt 

 step-like interruptions of the upper surface, a foot or more in 

 height, as if the sill had been broken by a series of subsidences. 

 The apparent fracture, however, is probably due to the irregularities 

 of the passage forced for itself by the molten rock, as it passed from 

 one line of horizontal joint to another through the heart of the 

 older sheet. 



The exposed surface of black glass on the top of the younger sill 



1 Op. cit. vol. xiv. (1858) p. 9; vol. xliv. (1888) p. 71. 



