CHAP. XVII 
MATERIALS ERUPTED— TUFFS, AGGLOMERATES 
79 
to “ consist of augite and hypersthene imbedded in and occurring amongst 
large plagioclase prisms. Some iron-ore is also present; the rock is a 
^^3. Tuffs cmd Agglomerates . — The fragmental materials, ejected from or 
lillino’ up the vents, vary from the finest compacted dust up to some of the 
coarsest agglomerates in this country. In general they consist mainly of 
detritus of andesite, and have been derived from the lilowing up of already 
consolidated masses of that rock. The fragments are usually angular, and 
range from minute grains up to blocks as large as a cottage. The tulls are 
often more or less mixed witli ordinary non- volcanic sediment, and as thy 
are traced away from the centres of eruption they pass insensibly into sand- 
stones and conglomerates. 
But while, as might be expected, the tuffs are most coinmoiily made up 
of del)ris of the same kind of lavas as those that usually form the sheets 
whicli were poured out at the surface, they include also bands of material 
derived from the destruction of much more acid rocks. Throughout the 
chain of the Ochil Hills, for example, in the midst of the bedded audesite- 
lavas, many of tlie thin courses of fine tuff consist largely of felsitic 
fraomeiits, with scattered felspar crystals. Tlie most remarkable examples 
of this nature, however, are to be met with at the great vent of the Lraid 
Hills in the chain of the Pentland Hills which runs south-westward froin 
it and in the Biggar volcanic district still further south. Tliese acid tuffs 
are «-enerally pale flesh-coloured or lilac iii tint, and compact iii texture, luit, 
like the felsitic lavas from which they were derived, they are apt to weather 
into yellow or buff “ claystone.s.” The finer varieties are so compact as to 
present to the naked eye no distinguishable grains ; they might be mistaken 
for felsites, and indeed, except where they coiitaiii recognizable fragments ot 
rock or broken crystals of felspar, can hardly be discriminated from theni. 
They consist of an exceedingly fine compacted felsitic dust. Here and 
there however, the scattered crystals of felspar and small angular frag- 
ments of felsite, which may be detected in them, increase in number uiffil 
tliey form the whole of the rock, which is then a brecciated tiifi or fine 
volcanic breccia, made up of different felsites, among which, even with the 
naked eye, delicate flow-structures may be detected. In these pale acid 
tuffs frao-ments of different andesites may often be observed, which inciease 
ill number as the rocks are traced away from the mam vents ol yuption. 
At my rerpiest my colleague, Mr. George Barrow, detennined the silica 
percentages in a few specimens which I selected as showing «o;ne ot the 
more chLacteristic varieties of these tuffs from the Braid and Pentland 
Hills. His results are exhibited in the following table ; 
1. Quarry above AVoodliouselee . • ■ - 
2. Soutli-weat side of Castlelaiv Hill . , tI h % 
3. Quarry on road, L mile N.E. of Swanston (Braid Hill vent) 
4. South-west side of Castlelaw Hill . • • • 
5. Castlelaw Hill . • • 
6. South side of White Hill Plantation 
Silica percentage. 
63-3 
73- 15 
74- 1 
75- 0 
76- 00 
90-00 
