252 C. LA.P WORTH OX THE MOFFAT SERIES. 



Immediately below the grey shales lies the thick series of black 

 shales already described, forming the first division of our first black- 

 shale group. Its coloured and nodular bands can be seen crossing 

 the cliff below the grey shales, and all its characteristic fossils can, 

 by the exercise of ordinary caution, be collected along the edge of 

 the cliff. 



These beds are here underlain by the strata missing from the 

 lateral gorge, viz. : — 



(o) A zone of black flags, in beds of .about 1 foot in thickness. 

 These rocks are remarkably resistant of atmospheric influences, and 

 form a jutting ledge running across the face of the cliff, overhanging 

 and protecting from degradation the more tractable beds imme- 

 diately below. 



Fossils are abundant, but of few species. The commonest form is 

 the strange Biplograptus vesicuhsus (Nich.), which, usually asso- 

 ciated with Climacograptus scalaris (His.), swarms in extraordinary 

 profusion in some of the beds. 



(c) The hard D. -vesicuhsus beds repose upon a similar thickness 

 of finely laminated black shales, weathering down into thin flakes of 

 a pale yellow colour. They contain a smaller proportion of carbona- 

 ceous matter than the overlying beds, and are softer and less 

 fossiliferous. 



They are characterized by the exclusive presence of Biplograptus 

 acuminatus (Nich.). 



The lowest band of this zone is very peculiar. It consists of 

 about a foot of tough slightly calcareous shale, weathering of a dark 

 brownish drab, or gingerbread-colour, and affording numerous 

 examples of Climacograptus scalaris in a state of high relief. 



At this point a sudden and most extraordinary change takes 

 place in the genera and species of the fossils of the Moffat Series. 

 Scarcely a single form of those collected by us in the strata already 

 described is met with in any of the beds below this line, which are, 

 however, like those above it, linked together by a large community 

 of organic forms. Here, therefore, is the chief palaeontological 

 break in the succession, and here, consequently, are we compelled to 

 draw our main line of demarcation in any natural classification of 

 the Moffat Series of this locality. 



The palaeontologist who makes a detailed study of the fossils of 

 that portion of the Moffat Series already passed under review will 

 soon assure himself of the fact that each species and variety of 

 Graptolite &c. has a definite range in the vertical succession of 

 strata. None pass from the lowest to the highest zone ; a few run 

 up through the majority of the beds ; several are common to two or 

 three subdivisions ; and the remainder are restricted to even nar- 

 rower limits. Some (and these are the most valuable for our 

 present purpose) are strictly confined to one or other of the minera- 

 logical zones we have indicated above. 



Further, it will be ascertained that the most prevalent forms in 

 these beds belong to the unilateral genera Monograptus (Geinitz) 

 and Bastrites (Barr.) of the family of the Monograptidse. Below 



