THE HIGHLAND BORDER ROCKS OF THE ABERFOYLE DISTRICT. 
183 
numerous large angular fragments of cherty shales, identical in character with 
certain shales of the underlying series. Blocks as large as 3 feet by l|- feet have 
been noted (see Plate V, fig. 4). Apart from the above exceptionally large fragments, 
the chief constituents of the breccia are vesicular palagonite, quartz, and smaller 
pieces of cherty mudstones and shales. 
The dominant material in nearly all the specimens is the palagonite, which occurs 
in fragments, the irregular shape of which has been determined by the fracturing of 
numerous vesicles and the consequent production of concave embayments on the 
margin of the grains (Plate III, fig. 2). The largest pieces are several millimetres 
in diameter, and all gradations are found down to minute particles showing 
characteristic “ ashen structur.” The palagonite is pale brown to greenish-grey in 
transmitted light, greyish-white in reflected light. The vesicles are infilled most 
frequently with green chlorite, less often with quartz and chalcedony, alone or in 
association with scaly chlorite. The glass has undergone complete alteration, and 
the only indication of the former presence of crystals is the occurrence of “ shapes ” 
of felspar microlites.. 
The fragments of sedimentary rocks show little variation in character. For the 
most part they are grey, greyish-black, and black shales and mudstones richly 
charged with rutile needles, and recalling at once the sediments of the Black Shale 
and Chert Series. Large blocks with thick white weathered crust are of the same 
type as the muddy radiolarian chert of the underlying series ; they resemble also 
the Glenkiln Flints * of the Southern Uplands. In thin section they show the 
characteristic “ shapes,” indicative of the presence of radiolarian tests as well as 
rhombs of ferrous carbonate. 
Quartz grains are always numerous, and in some specimens bulk quite as largely 
as the palagonite and sediments. They are often black in colour, and usually contain 
the inclusions characteristic of the quartz of granite rocks. A considerable propor- 
tion exhibit a unique type of recrystallisation, the origin of which is uncertain. 
Small separated portions, more or less circular, polarise differently from the quartz 
grain as a whole in such a way as to suggest that the quartz has undergone recrystal- 
lisation at isolated centres. There is sometimes a tendency for these areas to be 
arranged along a line or lines, and in one case they occur in such a way as to suggest 
that they have been developed along lines of conchoidal fracture ; more often, how- 
ever, their distribution is quite irregular. A point of some interest is that these 
unique quartzes appear to be confined to the basement breccias of the Margie Series ; 
they have not been found in the associated grits or in the Leny Grits immediately 
to the north. 
Felspar pebbles occur sparingly, and are absent in many specimens. In most cases 
they are orthoclase, less often perthite and acid plagioclase. 
Another type of pebble of not infrequent occurrence consists essentially of a fine- 
* Mem. Geol. Survey : The Silurian Rocks of Britain, vol. i, p. 41, 1899. 
TRANS. ROY. SOC. EDIN., YOL. LII, PART I (NO. 9). 
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