318 C. LAPWORTH ON THE MOFFAT SERIES. 



all except tile lowest zone of the Birkhill Shales, and with the more 

 sparingly distributed genera Diplograptus, Climacograptus, and 

 Retiolites (together with a few scattered forms of Crustacea and 

 Spongidse) constitute the whole of the fossils of the group. Con- 

 sequently, while there is no falling off in respect of individuals or 

 even species (many of the beds bearing favourable comparison with 

 the most prolific "horizons" of the Hartfell Shales), yet, when 

 contrasted with that afforded by the preceding divisions, the fauna 

 of the Birkhill Shales is strangely monotonous throughout. No 

 better proof could perhaps be adduced of our having clearly over- 

 stepped the limits of the great Llandeilo-Bala formation, where the 

 Graptolithina attain their maximum, and that we are now almost on 

 the threshold of those Upper Silurian rocks where these strange 

 old creatures disappear from our sight for ever, 



It will be superfluous to enumerate all the localities where this 

 division is exposed. This has been done already to a large extent 

 in the stratigraphical portion of the paper. As it immediately 

 underlies the greywackes, some of the zones of the group are neces- 

 sarily exposed in every spot where the Moffat Shales are visible, the 

 extent of our acquaintance with its strata and fossils increasing in 

 direct proportion as we approach its highest limit. It will be suffi- 

 cient for our purpose to define the characters of its successive zones 

 in some of the more symmetrical and fossiliferous sections, and, at 

 the same time, to indicate a few of the additional sections where 

 these facts may be verified and supplemented. 



(a) Lower Birkhill. (Pig. 29.) 



The thickness of this subdivision in the typical section of the 

 Main Cliff is about 60 feet. It is made up almost wholly of black 

 carbonaceous shales, occasionally diversified by intercalated seams 

 of variegated mudstones. The general lithological and palaeontolo- 

 gical features of the subdivision have been already so frequently 

 dwelt upon, that it is only necessary in this place to give a brief 

 summary of the characteristics of its three component zones. 



i. Zone of Diplograptus acuminatus (Nich.). — At the base of the 

 inferior subdivision of the Birkhill Shales lies a mass of shivery 

 shales, black- or blue-hearted, and weathering down into thin flakes, 

 superficially of a bright yellow colour. The basal seam of this band 

 is about 6 inches in thickness, consisting of a tough, flag -like shale, 

 slightly calcareous (see fig. 28). It is of a deep drab or gingerbread 

 colour where it is affected by the weather, and it yields numerous fossils 

 in admirable preservation. The details of the various sections of this 

 zone in Dobb's Linn may be gathered from our description of the rocks 

 of that locality. Its commonest fossils are Diplograptus acuminatus 

 (Kich.), D. vesiculosus (Nich.), rare, Climacograptus scalaris (His.), 

 Dimorphograptus elongatus (Lapw.). The same physical characters 

 mark this zone at Thirlstane Score, Scabcleuch, and Belcraig. At 

 the last-named locality it affords the same fossils. In the broken 

 section at Hartfell Diplograptus acuminatus occurs in some abund- 



