244 C. LAPWORTH ON THE MOFFAT SERIES. 



dency in these rocks to arrange themselves in broader and broader 

 zones, wholly made up of one or other of these elements, as we pass 

 over them from south to north, they are everywhere precisely simi- 

 lar, barren, monotonous, and uninviting. 



Lying at intervals among these unfossiliferous greywackes occur 

 thick bands of black carbonaceous shale, loaded with Graptolites, 

 and associated with beds of barren mudstone, grey, green, purple, 

 and occasionally pure white. These very distinctive strata run 

 longitudinally through the district in the direction of the strike, or 

 are exposed in long lenticular areas of small diameter. Being much 

 softer than the greywackes amid which they repose, they are more 

 easily destroyed by the action of the elements, and are usually 

 eroded into narrow valleys or form the beds occupied by the smaller 

 mountain-streams. The line of demarcation between them and the 

 greywackes everywhere gives rise to a prominent physical feature, 

 apparent even upon the turf-covered slopes. In the streams the 

 divisional line is in general strikingly marked by a picturesque cas- 

 cade, the water plunging over a precipice of grit into a deep black 

 hollow, worn out of the soft mudstones below. The strata proper 

 to the shale-bands are at once distinguished, by their colour, com- 

 position, and texture, from the rocks among which they lie ; and 

 where they cross the steeper ridges their place is marked by a deep 

 red gash or score in the mountain-side, strongly relieved against the 

 dark heather- clad slope, arid visible at a great distance. 



The black carbonaceous shales are highly pyritous, and the waters 

 that flow through them are all more or less impregnated with sul- 

 phate of iron. The mudstones themselves and the banks of the 

 streams for some distance below each exposure are normally stained 

 of a deep red or bright orange-colour by the mineral deposit from the 

 waters. The springs that rise among the shales, or are immediately 

 derived from them, afford the chalybeate waters for which the dis- 

 trict has long been celebrated. To the presence and efficacy of these 

 springs is owing the material prosperity of the flourishing town of 

 Moffat, the only place of importance in the district, and, at present, 

 the most fashionable watering-place in the south of Scotland. 



The title of Moffat Series, or Moffat Shales, by which these re- 

 markable deposits have long been known to geologists, is thus sin- 

 gularly appropriate ; and as these rocks attain here their fullest deve- 

 lopment, and at the same time are most satisfactorily exhibited, this 

 title is certain eventually to supersede all others as the general name 

 for all the Scottish graptolitiferous deposits of corresponding age. 



At least four distinct bands of the dark shales are traceable to 

 the south of the Moffat- Yarrow valley, and one of them is pro- 

 longed in a broken line far beyond the south-western limits of the 

 district. A larger number are apparent to the north of the central 

 valley ; but in none of these are the exposures so continuous or satis- 

 factory. 



Exception being made of a narrow strip of country in the neigh- 

 bourhood of St. Mary's Loch, where the strata have a southward 

 inclination, all the rocks of the district, greywackes and mudstones 



