CHAP. XXXVI 
STRUCTURE OF THE BAS ALT- PLATEAUX 
185 
rocks rather with the trachytes than the andesites. He remarks that “ in 
the apparent holo-crystalline character, the size and shape of the felspars, 
the sort of damascened appearance in polarized light, the finely scattered 
iron-ores and the presence of a pale green hornblende, possibly augite, in 
small, often complex, grains, these rocks much resemble the Carboniferous 
trachytes of the Garlton Hills in Scotland.” 
One of the most interesting lavas of the Tertiary volcanic series is the 
“ pitchstone-porphyry ” of the Scuir of Eigg. This rock, the latest known out- 
flow of lava in any of the volcanic areas of Britain, was formerly classed with 
the acid series. Microscopical and chemical analyses prove it, however, to be 
of intermediate composition, and to be referable to the andesites or dacites. 
It is more particularly described in Chapter xxxviii. 
Professor Judd, collecting the andesitic rocks as a whole (both lavas and 
sills), has grouped them into amphibole and mica-andesites, and pyroxene- 
andesites. 1 The thick lumpy and non-persistent sheets of these rocks 
sometimes found near the centres of protrusion of the gabbros and grano- 
phyres are probably sills. 
(e) Rhyolites . — In the Antrim plateau a group of rhyolite bosses occurs, 
some of which have been claimed as superficial lavas. In some cases it can 
be demonstrated that they are intrusive, and in no instance can they be 
decisively shown to have escaped in streams at the surface. It is probable, 
however, that some of these bosses did actually communicate with the outer 
air, for between the lower and upper group of basalts in this plateau, 
bands of rhyolitic conglomerate occur which may indicate the degradation 
of exposed masses of rhyolite. The description of these Antrim bosses 
will be given in Chapter xlvii., in connection with the acid eruptive rocks 
of the Tertiary volcanic series. 
2. Structure in the Field 
Passing now to the consideration of the lavas as they are built up into 
the plateaux, we have to note their distinctive characters as individual 
sheets of rock, and their influence on the topography of the regions in 
which they occur. Every tourist who has sailed along the cliffs of Antrim, 
Mull, Skye, or the Faroe Islands is familiar with the singular terraced 
structure of the great volcanic escarpments which stretch as mural precipices 
where these offered points of geological import I availed myself of them in tlie memoir published 
111 1888 in the Transactions of the Royal Society of Edinburgh. I have retained most of these 
citations in their place in the present volume, and have supplemented them by notes supplied 
to me from fresh observations by Mr. Watts and Mr. Harker. Professor Judd, in a series of 
valuable papers, has dismissed the general petrography of the Tertiary volcanic rocks {Quart. 
Jour. Geog. Soc. vols. xxxix. xli. xlii. xlvi. xlix.) 
1 Quart. Jowrn. Gcol. Soc. vol. xlvi. (1890), p. 356. Professor Judd has there described 
under the name of “propylites” various members of the volcanic series which he believes to 
have undergone alteration from solfataric action. 1 have not been able to discover any trace of 
such action, but I have found that the lavas of the plateaux assume a peculiar condition where 
they have been affected by large intrusive masses of granophyre or gabbro. (See postea, 
Chapter xlvi. ) 
