XXXIV. GEOLOGY OF DUMFRIESSHIRE. 



that the plants met with in the Millstone Grit, and the overlying true 

 coal measures, are specifically distinct from those which preceded them 

 in the carboniferous limestone and cementstone series. If this 

 generalisation should prove to be correct, then it is clear that the verti- 

 cal distribution of the fossil plants in the carboniferous system may be 

 of the greatest service in determining the horizon of the beds. Amongst 

 the plants obtained from the beds at the foot of Byre Burn, Archerbeck, 

 the Rowan Burn, and the Canobie coal field, and determined by Mr 

 Kidston, the following may be mentioned : — Sphenopteris L. and H., 

 Sph. obtusiloba Brongt., Staphylopteris sp., Neuropteris flexuosa Brongt., 

 Alethopteris lonchitica Schl , Pecopteris nervosa Brongt., Lepido- 

 phyllum lanceolatum. According to Mr Kidston, some of these forms 

 are never found below the horizon of the true coal measures, and it is 

 highly probable, therefore, that the Canobie coal field may represent 

 the true coal measures of the central valley of Scotland. The reddened 

 shales occurring to the south of the Canobie coal field yield Neurop- 

 teris flexuosa Brongt., a form confined to the true coal measures. 



Within the Silurian area, carboniferous rocks are met with in the 

 Thornhill and Sanquhar basins, filling hollows worn out of the old 

 Silurian tableland. At Closeburn and Barjarg there are beds of marine 

 limestone associated with sandstones and shales. Still further north, at 

 the south-eastern limit of the Sanquhar coal field, there are small out- 

 liers of the carboniferous limestone series, consisting of sandstones, 

 shales, and thin fossiliferous limestones. The latter rapidly thin out, and 

 the true coal measures rest directly on the Old Silurian platform. 

 From these data it would appear that in Upper Nithsdale the Silurian 

 barrier did not sink beneath the sea level till the latter part of the Car- 

 boniferous period. The Sanquhar coal field is about nine miles in 

 length, and from two to four miles in breadth. It contains several 

 valuable coal seams, and from the general character of the strata it is 

 probable that they are the southern prolongations of the Ayrshire coal 

 measures. 



Along the south-west part of the Sanquhar field, the strata are 

 traversed by three narrow dolerite dykes, which send out intrusive 

 sheets along the coal seams. The igneous rock is much decomposed, 

 having the same character as the white trap so common in the Ayrshire 

 coal fields. The coals are so altered by it as to be unworkable ; indeed 

 in some places they have been converted into columnar anthracite. 



The strata next in order are of Permian age, which are invariably 

 separated from all older rocks by a marked unconformability. They 

 occur in four separate areas — (i) at Moffat; (2) at Lochmaben and 

 Corncockle Moor ; (3) the Dumfries basin ; (4) the Thornhill basin. 

 The beds consist of red sandstones and breccias, the latter being 

 mainly derived from the denudation of the Silurian platform. Professor 

 Harkness, however, long ago noted the occurrence of fragments of 



