The Gcelentera m Relation to Geological Zones. 59 



of very much greater thickness, in areas at no great distance 

 from Moffat. The evidence, therefore, clearly points to the 

 inference that these dark-coloured, fine-grained mudstones 

 have accumulated at a very slow rate in deep water, on the 

 very outer margin of the main areas of deposit of sediments 

 of terrigenous origin. 



It may be as well to state here, for the information of 

 those who do not happen to be acquainted with the geological 

 details of the areas specially under consideration, that one or 

 more important unconformities exist in several areas in the 

 midst of rocks belonging to the same periods as those at 

 Moffat here under notice. Notwithstanding this, there does 

 not appear to be any clear evidence of a break in the con- 

 tinuity of the deposits in question. They can, it is true, be 

 grouped by means of their graptolites ; and the result obtained 

 by this means is as follows: — 



Lowermost Silurian Rocks '.^— 



C. BirkLill Shales. Thickness in Feet. 



2. Upper Subdivision, . . 46 



1. Lower Subdivision, . . 52 



The graptolites occurring in these may be grouped in 

 three : — (1) Those which have a wide vertical range, and 

 are therefore of but little value as zonal forms ; (2j those 

 which, while not absolutely confined to one definite horizon, 



^ I think that the term Llandeilo sliould be retained for the strata which 

 conformably succeed the Arenig Rocks, and lie below the unconformity so 

 commonly occurring on this horizon. 



