178 Proceedings of Royal Society of Edinburgh, [april 4> 
altered condition, they are well seen in the magnificent cliffs that 
extend westward from Pettico Wick Harbour. Between Colding- 
ham Bay and Callercove Point they are often much crushed, 
crumpled, twisted, shattered, shifted, and confused — the irregular 
puckerings and convolutions forming an interesting study. They 
are minutely cracked and fissured in all directions, the fine cracks 
and fissures being most frequently filled with white quartz, or with 
haematite and limonite. Where the strata are most highly con- 
torted, they frequently become seamed with a close, irregular net- 
work of small veins and mere threads of quartz, the meshes between 
which are often less than the 16 th part of an inch in diameter. 
Intrusive Felsite in the Silurian . — Through these excessively 
contorted rocks ramify here and there tortuous dykes and veins of 
felsite. The junction between those dykes and the rocks traversed 
by them is generally well marked. But here and there it is much 
confused — the fine-grained greywackes being baked and altered, and 
having the same pale-grey colour as the felsite, so that the line of 
parting between the two can hardly he distinguished by the un- 
assisted eye. Under the microscope, however, the crystalline and 
fragmental rocks are readily discriminated. These felsites are con- 
fined to the Silurian areas. Nowhere, so far as I saw (and I tra- 
versed a considerable area round Coldingham and Eyemouth), do any 
of the felsitic intrusions penetrate rocks of Old Bed Sandstone age. 
The rock of these dykes is grey or pale pinkish-grey in colour, 
compact, sparingly porphyritic, with microscopic crystals of quartz 
and felspar, — having, in short, the appearance of typical felsite. 
Here and there veins of white quartz seam the dykes. Viewed 
under the microscope the rock exhibits a microfelsitic base, 
scattered through which are abundant small crystals of orthoclase, 
and a few larger ones of the same mineral. Oligoclase also appears 
in well-developed crystals, and to the same species some of the 
smaller crystals of felspar seem to be referable. Both felspars show 
alteration into saussurite, but this is most frequently the case with 
the orthoclase. Crystals of quartz, often much corroded, but not 
unfrequently showing well-defined pyramidal forms, are common ; 
and they generally contain fluid cavities, sometimes in very great 
abundance. One or two thin spicules of a dichroic mineral, 
probably mica, were seen, but only in one section. The most 
