320 C. LAPWORTH ON THE MOFFAT SERIES. 



ance; but neither there nor in the great section at Craigmichan 

 Scaurs can the zone he identified with certainty. 



ii. Zone of Diplograptus vesiculosus {Nidi.). — This zone with its 

 hard thick-bedded black flagstones, everywhere so prominent among 

 the surrounding strata and so doggedly resistant of those atmospheric 

 influences that have crumbled the underlying shales to powder, has 

 been so frequently alluded to in the foregoing pages as to preclude 

 any special description in this place. Next to the typical section at 

 Dobb's Linn it is exposed in the Score above Thirlstane Burn, and 

 in the fine section at Belcraig. Strictly speaking, it everywhere 

 merges imperceptibly into the lowest beds of the succeeding zone, 

 which possess corresponding lithological characters and yield, gene- 

 rally speaking, the same fossils. 



The fauna of the D. -vesiculosus zone at Dobb's Linn includes the 

 following species, which occur in abundance on a few distant horizons, 

 the intervening beds being wholly destitute of organic remains : — 

 Dialog vastus vesiculosus (Kich.), Olimacogrctptus scalaris (His.), C. 

 innotatus (Nich.), Monograjptus tenuis (Portl.), M. attenuatus (Hopk.), 

 Dimorphogrcijptus elongatus (Lapw.). 



iii. Zone of WonogYsuptus gregarius {Lapw.). — The flags of the D.- 

 vesiculosus zone graduate upwards into a great succession of black 

 flags, shales, and mudstones. Among them abound seams and beds of 

 coloured clay or mudstone, blue, grey, purple, or black. These are 

 much softer than the flaggy rock, and are worn down into deep 

 grooves and channels on the face of every section. At intervals 

 occur bands and lines of nodules of calcareous ironstone, forming 

 prominent ribs and ridges, rising conspicuously amid the more 

 tractable beds around. The whole group abounds with iron pyrites, 

 either included in the shales themselves, or forming seams and 

 bunches of bright yellow concretions and crystals. The waters that 

 trickle over these beds take up this mineral in their course, streaking 

 and staining the whole mass of beds of a deep rusty yellow with 

 a subsequent deposit of oxide of iron. 



At Dobb's Linn a bed of concretionary and calcareous ironstone, 

 near the centre of the zone, forms a convenient boundary between 

 its lower and upper divisions. Below that line the dark carbona- 

 ceous shales are hard and flag-like, and show few of the coloured 

 courses. Above it the beds are thinner, and the mudstone bands 

 are excessively numerous. The lower division is here 20 feet in 

 thickness, and the lower 15 feet. This thickness is greatly in excess 

 of that of any of the remaining zones of the Birkhill Shales ; and it 

 will also be seen that the fauna is larger and more varied. There 

 can be no doubt, however, that these beds constitute one single 

 physical mass, which, neither mineralogically nor zoologically, is 

 it possible satisfactorily to subdivide. 



This group stands related to the main mass of the Birkhill forma- 

 tion in a corresponding position to that occupied by the zone of 

 Dicellograptus Clingani with respect to the Hartfell Shales. It em- 

 braces, namely, a large proportion of the most distinctive beds of the 

 formation and includes a majority of its most characteristic fossils. 



