840 DR B. N. PEACH AND DR J. HORNE ON 



strata consists of reddish pebbly sandstones, composed mainly of materials derived from 

 the Silurian tableland, but at certain localities, as for instance on the west slope of 

 Whita Hill, there is an admixture of debris of igneous rocks resembling the Lower Old 

 Red Sandstone andesites of the Cheviots. These are overlaid by red carious weathering 

 sandstones, with occasional pebbles ; and towards the top the calcareous matter is 

 aggregated in knots and lenticles, evidently representing the horizon of the cornstone 

 which occurs near the top of this formation. Indeed, near Riccarton, in the north-west, 

 this zone does occur, where it is often accompanied by a lenticular red chert band. No 

 fish-remains have been found in these strata near Langholm, but in the sandstones of 

 Dinley Burn, near the Dinley Spout, a tributary of the Hermitage Water, scales of 

 Holoptychius nobilissimus have been met with. 



Lower Carboniferous Rocks. 

 i. The Volcanic Rocks of the Tarras Water and Birrenswark. 



In the district now under consideration the Upper Old Red Sandstone strata are 

 everywhere surmounted by a zone of contemporaneous volcanic rocks that form a well 

 marked horizon in the geological sequence. The lava flows, which are usually slaggy 

 and much decomposed, are of a basic character, ranging from olivine-basalts to andesitic 

 basalts. Hardly any tuff or volcanic agglomerate has been observed in this volcanic 

 zone. As might naturally be expected, few vents filled with volcanic agglomerate pierce 

 the strata of a lower geological horizon to the north of the volcanic platform. But in 

 the area north of the Tarras and Ewes Waters (sheet 17 — one-inch) numerous plugs or 

 stocks of intrusive igneous rocks, of intermediate or acid types, resembling those of the 

 Eildons near Melrose, appear within the Silurian area. An excellent example of a plug 

 of andesitic basalt rising through the Upper Old Red Sandstone underlying the volcanic 

 zone is to be found on Arkleton Hill, six miles north of Langholm, in the basin of the 

 Ewes Water. 



This volcanic zone forms a narrow fringe round the Carboniferous area, though the 

 outcrop is much interrupted by faults, some of which are of considerable magnitude. It 

 has been traced for several miles alone; the eastern margin of the Silurian inlier near 

 Riccarton. From Dinley on the Hermitage Water, it has been followed at intervals 

 across the heights to the Whita Hill near Langholm, thence westwards by Waterbeck 

 and Middlebie to Birrenswark, and beyond the Dumfries basin of the New Red Sandstone 

 it reappears in the district of Kirkbean. 



ii. The Whita Sandstone. 



The volcanic zone of Birrenswark and the Tarras Water is overlaid by a group of 

 sandstones which are characteristically developed on the Whita Hill, about half a mile 



