THE GEOLOGICAL STRUCTURE OF THE CANONBIE COALFIELD. 841 



east of Langholm. At the base, the beds consist of pink pebbly sandstones, the pebbles 

 being well rounded and composed of vein quartz. These pass upwards into yellowish 

 or grey mottled gritty sandstones, with marked false-bedding, and containing clay galls 

 and specks of decomposing carbonates. Near the base of this type, and above the 

 pinkish sandstone, there are occasional lenticular beds of ochreous cementstone. The 

 highest members of the group consist usually of coarse, mottled, carious weathering 

 sandstones, with decomposed, rusty carbonates. Occasionally beds of greenish shale 

 and cementstone are intercalated in the series near the top. It is highly probable that 

 the galls of shale and particles of carbonate found in the sandstone have been derived 

 from the erosion of the beds of shale and cementstone that appear to have been laid 

 down during pauses in the deposition of coarser sediment. The thickness of this zone 

 ranges from 600 to 700 feet. 



In the Esk south of Langholm, from Longwood to below Broomholm — a distance of 

 about a mile — this arenaceous group may be traced in natural sequence above the 

 volcanic zone of the Tarras AVater and Birrenswark. Thence they stretch northwards, 

 forming the tops of the Fells from the Whita Hill to Dinley Fell near the Hermitage 

 Water — a distance of nearly twelve miles. Across the Hermitage Water they may be 

 followed north-eastwards as far as the Whitterhope Edge, where they are truncated by 

 a fault. A second outcrop of this group of sandstones appears in the upper part of the 

 valley of the Liddel and to the east of the Silurian inlier at Riccarton. From Lid del 

 Castle, about three miles above New Castleton, it extends north-eastwards, flanking the 

 rocks of the Birrenswark volcanic zone, across the Biccarton, the Dawston, and the 

 Caddroun Burns, beyond which the sandstones are obliquely truncated by a fault. Along 

 this outcrop the Whita sandstones cannot exceed 300 feet in thickness, and they appear 

 to thin out towards the north-east. 



iii. Cementstone Group. 



This subdivision consists mainly of green, blue, and grey mudstones and sandy 

 shales, with bands of impure muddy limestone and cementstone, with occasional inter- 

 calations of coarse grey calcareous sandstone. Near the top there is a zone of true 

 marine limestone. In the Langholm district the thickness of this group varies from 

 1200 to 1500 feet. 



The cementstone group of Eskdale and Liddisdale was evidently deposited along- 

 shore under estuarine conditions, the mudflats being suitable for the growth of lamelli- 

 branchs, which occur in profusion in certain beds. Indeed, in some bands one species 

 of Modiola appears, to the exclusion of every other form. The characteristic shell is 

 Modiola Macadami and its varieties, but other bivalves also occur, though less abun- 

 dantly. A good section of these beds exposed in the cliff of the Liddel Water opposite 



