C. LAPW0RTH ON THE MOFEAT SERIES. 



321 



Below the central nodule-band the commonest species are :- 



Eastrites peregrinus {Barr.). 

 Monograptus Sandersoni (Lapw.). 



cyphus (Lapw.). 



tenuis (Port/.). 



• communis (Lapw.). 



triangulatus (Harkn.). 



- gregarius (Lapw.). 



Monograptus concinnus (Lapw.). 

 Diplograptus tamariscus (Nich.), 



vesiculosus (Nich.). 



physophora (Nich.). 



moclestus (Lapw.). 



Olimacograptus innotatus (Nich.), 



Above the nodule-band many of these recur and are associated 

 with : — 



Monograptus leptotheca (Lapw.). 



lobiferus (M'Coy). 



Dawsonia campanulata (Nich.). 



Discinocaris Browniana (Woodiv.), 

 Diplograptus folium (His.). 

 insectiformis (Nich.). 



Some of these forms have a very restricted vertical range in the 

 beds of this zone. Monograptus Sandersoni (Lapw.) arid M. fim- 

 briatus (Nich.) are unknown above the central line. Monograptus 

 triangulatus (Harkn.) occurs only in the neighbourhood of the nodule- 

 band. Neither Rastrites peregrinus nor Diplograptus vesiculosus 

 reach the summit of the group. 



All the localities noted as exhibiting the black and variegated beds 

 of this zone afford also, as a rule, their characteristic fossils. 

 Nowhere is it possible to point out any marked distinction between 

 these beds at Dobb's Linn and their representatives elsewhere, 

 except, perhaps, along the southern band of Ettrick and Glen- 

 kiln, where they are usually greatly hardened, have a somewhat 

 slaty structure, and show only a small proportion of the variegated 

 mudstones. 



In the last three or four feet of the beds of this zone, as exposed 

 at Birkhill and in the Thirlstane Score, we have evidence of the 

 commencement of those conditions which led to the production of 

 the thick overlying mass of grey flags, in the presence of numerous 

 hard seams of grey shale intercalated among the normal fossiliferous 

 dark shales and mudstones. The terminal beds afford always the 

 best-preserved fossils of the zone, but they are rare and of few 

 species. 



(b) Upper Birkhill. (Fig. 30.) 



The finest and, in many respects, the most typical section of this 

 subdivision is seen in the gorge and corrie immediately below the 

 falls of Dobb's Linn. It here embraces about 77 feet of barren grey 

 flaggy mudstones, and includes three distinct groups or horizons of 

 black fossiliferous shales. The base of the group appears well- 

 defined, being marked by a sudden lithological break. Actually, 

 however, its first fossiliferous zone has little to distinguish it from 

 the underlying beds. The grey shales near the summit of the group 

 become harder and more flag-like as we ascend the succession, and 

 there is an almost insensible lithological transition into the basement 

 beds of the great greywacke formation of the Gala group. From 

 the circumstances of the case, the final line of demarcation is neces- 



