360 SIR A. GEIKIE ON THE TERTIARY [May 1 896,- 



into impure coal. Immediately overlying this coaly layer lies a- 

 sheet of prismatic vesicular basalt, followed by another with an 

 exceedingly slaggy texture. 



Lenticles of shale and mudstone likewise occur in the heart of 

 the finer parts of the conglomerate, especially towards the top, as 

 may be seen in the section exposed beneath the basalt behind the 

 first cottage west from Canna House. One of the most interesting 

 layers in this section is a seam of tuff varying up to about 2 

 inches in thickness, which lies at the top of the lenticular band of 

 tuffs and shales, and immediately beneath the band of basalt- 

 conglomerate, on which a basalt, carrying a vesicular band near its 

 bottom, rests. Traced laterally, the dark brown tuff of this seam- 

 gradually passes into a series of rounded bodies and flattened shells 

 composed of a colourless mineral which has evidently been developed 

 in situ after the deposition of the tuff. Mr. Harker's notes on thin 

 slices made from this band are as follows : — 



'This is a rusty brown, dull-looking rock, rather soft and seemingly 

 light, but too absorbent to permit of its specific gravity being tested. 

 The dark brown mass is in great part studded with little spheroidal; 

 bodies, ^ to ^ inch in diameter, of paler colour, but the larger ones 

 having a dark nucleus. In other parts larger flat bodies have been 

 formed, as if by the coalescence of the spheroids, extending as 

 inconstant bands in the direction of lamination for perhaps | inch y 

 with a thickness of J_ inch or less. The appearance is that of a 

 spherulitic rather than an oolitic structure. 



' A slice [6658 a] shows the general mass of the rock to be of an 

 extremely finely divided but coherent substance of brown colour, 

 which can scarcely be other than a fine volcanic dust composed of 

 minute particles of basic glass or ' palagonite ' compacted together. 

 Scattered through this are fragments of crystals recognizable as 

 triclinic and perhaps monoclinic felspar, green hornblende, augite y 

 olivine (?), and magnetite, usually quite fresh. 



' The curious spheroidal and elongated growths already mentioned 

 are better seen in another slide [6658 b], where they occupy the 

 larger part of the field, leaving only an interstitial framework of 

 the brown matrix. The substance of the little spheroids is clear, 

 colourless, and apparently structureless. The centre is often occu- 

 pied by an irregularly stellate patch of brown colour, and sometimes 

 cracks tend to run in radiating fashion, but these are the only 

 indications of radial structure. The outer boundary is sharply 

 defined, and where the slice is shattered the spheroids have separated 

 from the matrix. The matrix is darker than in the normal rock r 

 being obscured by iron oxide which we may conceive as having beert 

 expelled from the spaces occupied by the spheroids. The little 

 crystal-fragments are enclosed in the spheroids as well as in the 

 matrix, but there is no appearance of their having served as starting- 

 points for radiate growths. The flat elongated bodies are like the 

 spheroids, with merely the modifications implied in their different 

 shape. 



4 The identity of the clear colourless substance seems to be rather 



