C. LAPWOBTH ON THE MOFFAT SEBIES. 



317 



yellow, or white niudstone, never more than 2 or 3 inches in 

 thickness. 



The arrangement of the strata of this well-marked zone has been 

 given already in our description of the physical structure of the 

 ground at Dobb's Linn. (p. 253). The fossils obtained belong ex- 

 clusively to the species : — ■ 



Clirnacograptus bicornis, var. tubercu- 



latus, Nick. 

 scalaris (His.). 



Diplograptus truncatus (Lapw.). 

 Dicellograptus anceps (Nick.). 



The same zone is exposed in Eiskinhope Burn (fig. 10) under the 

 same general aspect, and yielding the foregoing Graptolites in some 

 abundance ; but it is more satisfactorily exhibited in the beautiful 

 section in Black Grain (fig. 12), where the dark seams are very 

 prominent, and the fossils are numerous and well preserved. 



§ III. Birhhill Shales. 



The third or Birkhill division of the Moffat Series comprises all 

 the fossiliferous strata that intervene between the B .-anceps zone of 

 the Upper Hartfell Shales and the coarse grits and flagstones of the 

 Gala group. In the typical section of Dobb's Linn, the total 

 thickness of the division is about 140 feet. This thickness is pro- 

 bably exceeded in the district lying to the south of the Moffat 

 valley ; but in none of the sections there exhibited are its strata in 

 such an attitude as to admit of exact admeasurement. 



In their mineralogical characters the beds of this division, while 

 bearing a decided resemblance to those of the other divisions of the 

 Moffat Series, at the same time possess many marked peculiarities. The 

 dark carbonaceous shales of this division never show the slaty, close- 

 grained texture or the plate-like structure of those of the subjacent 

 Hartfell group. They are normally soft, irregularly laminated, and 

 split under the hammer into small slabs or flakes, with a ridged and 

 uneven surface. The unfossiliferous muds tones are quite as distinct, 

 no longer weathering down into yellow prismoid fragments, but 

 forming thin flag-like sheets of a greyish-green or deep purple 

 colour, and resembling in all their essential features those of the 

 corresponding strata of the succeeding Gala group. 



The physical peculiarities of the rocks of the Birkhill division are 

 accompanied by far more important distinctions in the facies of its 

 included fauna. Of the numerous genera of compound Graptoloidea 

 which gave such a varied character to the fauna of the Glenkiln 

 Shales, and many of which have accompanied us in our upward 

 progress into the typical beds of the Hartfell division, not one 

 passes up into the Birkhill Shales. Here, on the other hand, 

 the extraordinary prevalence of Monograptidse upon every zone is in 

 striking contrast to what occurs in the inferior division, where not 

 the slightest trace of any form of this family has ever been detected. 

 The two genera Monograptus and Rastrites swarm abundantly in 



