392 PROFESSOR GEIKIE ON THE 
Weydale. They are dark-grey, hard, fissile, calcareous, and partly bituminous 
strata, capable of being split into large, clean, smooth flags, an inch or less in 
thickness and of uncommon toughness. The zone which emerges on the coast 
below Castletown is probably the lowest, from which the best, that is, toughest 
and thinnest flagstones can be procured. An analysis of four specimens from 
the Castlehill quarries by Dr Hormann, to whom they were submitted by Sir 
R. Murcuison, gave the results quoted below.* 
From Castletown to Thurso the succeeding strata are admirably laid bare, 
both in vertical sections along the low and easily scaled cliffs, and in horizontal 
ground-plan on the beach, where-broad sheets of the rock slope gently into the 
sea. The prevalent dip continues north-westerly, with one or two gentle rolls, 
and the angle of inclination seldom exceeds 8°. By an observer coming fresh 
from the more massive flagstones of the lower group the first features to be 
noticed are the thin-bedded, almost shaly character of the strata here, their 
pale-yellow and green colours on weathered surfaces, and their evidently cal- 
careous composition. On closer examination they are found to consist of fissile, 
calcareous, grey, hard flagstones, green, grey, and brown calcareous (and fre- 
quently bituminous) shales, with thin bands of calcareous gritty sandstone and 
argillaceous limestone (‘‘calmy limestone”), seldom more than a few inches in 
thickness. The influence of weathering generally produces a yellow or greenish- 
grey tint on the surface of the rocks, while at the same time it reveals the 
exceedingly fine lines of deposit on many of the flagstones. Even when split 
into smooth sheets an inch or less in thickness, these hard, tough layers show 
on their yellow, weathered edges successive paper-like but mutually adherent 
lamine. The general thin-bedded, shaly, and calcareous characters of these 
strata recall the aspect of some of the lower parts of the Carboniferous system 
in the south of Scotland. 
The next feature to engage the attention of the observer is probably the 
extraordinary abundance of ripple-marked surfaces and sun-cracks. Though 
these markings abound also in the lower flagstone group, it is here that they 
attain their greatest development. Surfaces of flagstone or shale, many square 
* “Quart. Journ. Geol. Soe.” vol. xv. p. 402. 
Salts of 
ines Oxide of Carbonate | Organic Water Magnesia 
= Tron and = Loss at >! Total. 
Tnselsibte ‘Alana, of Lime. Matter. 100° bs pero 
No. 16, Top Flag, ; 68:40 10:21 10:93 3°88 0°42 6°16 100:00 
No. 7, Middle Flag, : 69°45 11°50 10°66 5:79 0-40 2-20 100:00 
Bituminous Shale, : 69:96 8:15 7:72 10°73 0°53 2°91 100:00° 
No. 1, Bottom Flag, _. 61:39 4°87 21-91 3°40 0:20 8:23 100-00 


