50 Proceedings of the Royal Society of Edinburgh. [Sess. 
The conditions under which the series accumulated were apparently not 
greatly different : an estuarine and deltaic phase with lagoon and even 
terrestrial conditions prevailed, broken only by the occasional marine phases 
which are so typical of all coal-fields from Carboniferous to Tertiary. 
Speaking broadly, the Carboniferous areas in Britain are in the form of 
large and often irregular basins among older strata — broadly synclinal areas, 
in fact ; while in the corresponding anticlinal areas the Carboniferous rocks 
have been removed by denudation. 
Oil-shales in Scotland, however, are not only confined to certain groups 
in the Carboniferous formation, but to certain regions only where these 
groups are developed. Thus, though the Coal Measures persist through the 
west of Scotland, in the Lanarkshire and Ayrshire basins the Calciferous 
Sandstone series does not contain oil-shales. It is in fact almost entirely 
in the Lothians that oil-shale has been worked in Scotland, and even in 
this restricted area the Oil-Shale group does not contain workable seams 
throughout. 
The workable shale-fields are divided by the broad anticlinal area of 
the Pentland Hills, which, running north-east and south-west, cut off the 
Midlothian coal-field from the greater shale-fields to the north-west. On 
the north-western margin of the Midlothian coal-field, a steep and narrow 
syncline, shale seams have been worked near Straiton and Carlops, where 
the strata are vertical or dipping very steeply. Where the same strata 
emerge on the eastern and south-eastern side of the syncline there are no 
oil-shales. 
The broadly anticlinal ridge of the Pentlands, though very steep on the 
south-eastern side, is much more gently inclined on the north-western, and[ 
here a great triangular area from Blackness to Tarbrax and from thence to 
Dalmeny is characterised by the presence of oil-shales. To the westward 
this area is bounded by the overlying Carboniferous Limestone series, 
beneath which workable oil-shales die out. Thus only a narrow strip on 
the steep flank of the Pentland anticline and a broad area on the gently 
inclined flank furnish shale-fields. The broad triangular area is much 
complicated by minor folds, and faults of later date, sharp anticlines, and 
dome structures of no great size are frequent, and igneous intrusions of 
Carboniferous age, both sills and irregular masses, are numerous. 
A somewhat more detailed account of the flexures is called for, and for 
this purpose a quotation from the memoir on The Oil-Shales of the Lothians 
will suffice : — 
“ The flexures are usually normal, but in a few cases the strata become 
vertical or slightly inverted. 
