464 Proceedings of Royal Society of Edinburgh. [sess. 
cementing material is very largely obscured by a dusty-grey or 
brown amorphous substance and by black carbonaceous matter. 
Where the grains are fairly large and well packed this forms a 
sort of network, in the meshes of which lie the quartz grains. 
Where the grains are not so close it is more distinct, and under 
crossed Nicols has a crypto-crystalline appearance, practically 
identical with that of chalcedony. A few chlorite flakes occur 
in it here and there. Small veins traverse the section, some con- 
taining calcite, others a fine quartz mosaic. Bedding is not seen 
in hand specimens, and in many places in the field it cannot 
be made out either, the rocks having a massive character, but 
much traversed by cracks and faults, shattering them into irregular 
masses. 
In other places, again, the bedding is distinct, or even marked. 
Where this is the case the individual beds vary in thickness 
from a few inches to several feet. Very often the bedding has 
a contorted, or rather, wavy character, more conspicuous when 
viewed from some distance off. 
On some of the cliffs faulting is very marked, which has 
probably given rise to the general shattered condition of the 
rocks. Most of the faults noted are strike-faults. When the 
faults are not so much in evidence, the rock shows in places well- 
marked jointing, often very difficult to distinguish from bedding 
planes. 
Varieties of the greywacke occur. These are of very local 
occurrence, and are not usually sharply defined, but shade off 
imperceptibly into the common type. The following are the 
principal varieties : — 
1. Greywacke conglomerate. Contains rounded quartz pebbles, 
not usually larger than J in. in diameter, and pieces of dark slate 
or shale, rounded or flattened angular laminae, up to J in. in length. 
This is an extremely hard, tough rock, intimately pervaded by the 
siliceous matrix, so that the grains seem to fade into each other and 
into the cementing material, instead of having sharp outlines. When 
fractured, the component pebbles break across, but on natural 
weathered surface the matrix gives way sooner, leaving the indi- 
vidual pebbles sticking out as in a conglomerate. A microscopic 
section shows that the allothigenic or derived materials are practically 
