C. LAPWOETH ON THE MOEEAT SERIES. 307 



district. In lithological character its beds resemble those of the 

 underlying Glenkiln division, the black shales having a correspond- 

 ing plate-like or slaty structure, and the pale mudstones weathering 

 down into similar small prismoid fragments of a deep grey or orange- 

 yellow colour. On the other hand, the dark fossiliferous shales are 

 more numerous, and are grouped together into a single mass, or form 

 narrow seams of a few inches in thickness among the barren mud- 

 stones. A further distinction is shown in the absence of the pecu- 

 liar ribs of hard grey siliceous flagstones so conspicuous in the 

 underlying group. 



Mineralogically the line of demarcation between the Glenkiln and 

 Hartfell divisions is well marked, and is recognizable at a glance 

 wherever the two groups are seen in conformable juxtaposition. The 

 thick mass of pale flags and mudstones last described constitutes the 

 concluding portion of the Glenkiln beds ; and the base of the Hart- 

 fell division is formed by the black shales, in which fossils begin to 

 reappear in abundance. The upper boundary-line is quite as dis- 

 tinct, a similar group of barren mudstones being intercalated between 

 the fossiliferous Hartfell strata and the basal beds of the Birkhill 

 Shales. 



Palseontologically there is no actual break between the Glenkiln 

 and Hartfell divisions. The fossils of the lower division die out 

 one by one as we ascend the succession, and are as gradually re- 

 placed by others till the change of fauna is complete. Thus, although 

 the two successive faunas are linked together by a large community 

 of genera and species, yet, on the other hand, the typical beds of the 

 two formations have scarcely a single fossil in common. 



At the summit of the Hartfell Shales, on the contrary, the palse- 

 ontological break is almost complete. Two Graptolites only are 

 supposed to pass up* into the overlying Birkhill formation, and even 

 these are represented by very distinct varieties. 



The fauna of the Hartfell Shales is characterized chiefly by the 

 extraordinary predominance of the genera Dicellograptus, Pleuro- 

 grcvptus, and Diplograptus. Dicellograptus is a survival from the 

 Glenkiln fauna, in which, however, it plays a very insignificant part. 

 Here, on the other hand, its individuals swarm in countless multi- 

 tudes ; on many horizons the surface of the shale-beds is almost 

 hidden from sight by the complex network formed of the entangled 

 branches of thousands of examples. 



Diplograptus is common to all the divisions of the Moffat Series ; 

 but in none is it so prolific as in the Hartfell Shales, where it is fully 

 equal to Dicellograptus in variety of form and abundance of indi- 

 viduals. 



Pleurograptus, with its intimate ally, Aniphigraptus, is strictly 

 confined to the Hartfell division, and even within it rarely trans- 

 gresses the limits of the zone to which it gives its name. 



There is an extraordinary mortality among the Graptolites in this 

 division. Not only do the peculiar types, Pleurograptus and Amphi- 

 grajHus, arise, culminate, and decay within the formation itself, but 

 Dicellograptus, Bicrauograptus, La siog rectus, Glossograptus, and 



