1916-17.] The Hurlet Sequence and the Abden Fauna. 207 
Succession in the North-west o£ England * has correlated the limestones of 
Northumberland and North Cumberland from the Oxford Limestone to 
the Acre Limestone with his Upper Sub-Zone of the Dibunophyllum Zone 
in the North-western Province, which he has further correlated with the 
D 2 Zone of the South-western Province and Midlands. Though we have 
not had the privilege of examining Professor Garwood’s ground, it seems 
to us that his various bands, sub-zones, and zones rest upon a thoroughly 
secure stratigraphical and palaeontological basis, presenting phenomena 
similar to that which we have met with in the region just described. 
In the present state of our knowledge of Carboniferous stratigraphy 
and palaeontology we consider it particularly undesirable that too much 
reliance should be placed upon the appearance and disappearance of certain 
forms over limited areas, or that too much importance should be attributed 
to varieties or mutations supposed to represent an evolutionary series,, 
and assumed to be of value as time indices. The late Dr Vaughan and his 
co-workers have as a rule based their correlations upon the general char- 
acter of certain faunal assemblages, and have been enabled thereby to trace 
certain horizons from place to place, and it is upon this principle that the 
marine strata of the Scottish Carboniferous System from the Hollybush 
Limestone up to the Upper Limestone Series have been correlated with 
the Upper Sub-Zone D 2 of Vaughan’s Dibunophyllum Zone. That there 
is a close resemblance between the faunal assemblages of the two areas 
cannot be disputed, but that these indicate anything more than a similarity 
of physical conditions we consider has yet to be proved. 
XV. Physical Conditions of Deposition. 
Judging from the evidence that has been adduced in the foregoing part 
of this paper, it will be seen that the physical conditions which prevailed 
during the deposition of that part of the Carboniferous Formation which 
lies below the Blackbyre Limestone presented three well-marked and 
diverse types, namely, that seen in the West of Scotland, that seen in the 
Lothians, and that seen in East Fife. But when we reach the horizon 
of the Hurlet Limestone, and from that up to the Top Marine Band, a 
fairly uniform condition of deposit seems to have prevailed over Central 
Scotland. 
The Hollybush and Blackbyre Limestones where typically developed 
are full of such clear water marine forms as foraminifera, corals, crinoids, 
echinoids, polyzoa, lamellibranchs, gasteropods, cephalopods, brachiopods, and 
* “ The Lower Carboniferous Succession in the North-west of England/’ Quart. Jour. 
Geol. Soc., vol. lxviii, p. 449. 
