THE HIGHLAND BORDER ROCKS OF THE ABERFOYLE DISTRICT. 
185 
remarkably fresh, but occasionally the orthoclases show advanced se'ricitisation. 
Clastic muscovite appears in every specimen examined. In a few cases it is accom- 
panied by biotite (usually partially or completely chloritised), either in parallel inter- 
growth or in separate fragments. Zircons, sometimes highly coloured and pleochroic, 
and often zoned, are constant accessory constituents, and are often very abundant. 
Dark green tourmaline is rarely absent. 
The composite grains include cpiartz-orthoclase, quartz-biotite, quartz-muscovite, 
orthoclase-biotite, orthoclase-zircon, etc., all derived doubtless from the disintegra- 
tion of granites, fragments of black shales and sandy mudstones, and in several 
instances pieces of decomposed volcanic rocks, in most cases spilites. 
The matrix is very uniform in character, consisting of small detrital fragments 
of the same minerals as the larger grains, together with what must have been a fine 
argillaceous paste or mud. The latter, as in the basement breccias, is now repre- 
sented by authigenous minerals in the form of flakes of colourless micas, green scaly 
chlorite, rutile needles, and pyrites. Locally the matrix has undergone silicification, 
and subsequent partial calcification is sometimes met with. The Margie Grits of the 
Aberfoyle district, however, do not show the rich development of carbonates, which 
is given by Mr Barrow as a distinctive character of the grits of the Forfar- 
Kincardine area. # 
Shales . — The fine-grained greyish-green shales and mudstones of the Margie 
Series consist of authigenous scaly chlorite, colourless micas, iron oxides, and 
occasional rutile needles, along with small clastic grains of quartz, muscovite, and 
biotite. They are well exposed in the Corrie Burn section. 
Limestone . — The limestone occurs in the old lime quarry north of Upper Dounans, 
where it is faulted down between the local serpentine and the Lower Old Red Sand- 
stone (see text-fig. 10). It is a bluish crystalline limestone, appearing yellow or 
brown on weathered surfaces. It is usually much brecciated and veined with white 
calcite. Its chief interest centres round the fact that it has been proved to be in 
places highly fossiliferous. 
The fragments of sedimentary rocks included in the Margi'e Grits recall at once 
the sediments of the underlying series. The larger inclusions in the basement 
breccia must either have fallen from cliffs adjoining the Margie sea, or be due to 
volcanic explosions on the sea floor ; the abundant palagonite fragments point to a 
recrudescence of volcanic activity at the beginning of Margie times. The fact that 
pieces of the spilite lavas and the underlying black shales are of common occurrence 
both in the basement breccias and the grits indicates a noteworthy amount of 
denudation of the Lower Series. The main constituents of the grits, however, show 
clearly that they were derived in large measure from the denudation of an area of 
granitic rocks, t 
* Quart. Journ. Geol. Soc., vol. lvii, p. 329, 1901. 
t This point is discussed more fully in tlie comparison of the Margie Grits with the Leny Grits on page 200, 
