Vol. 67.] THE CARBONIFEROUS SUCCESSION IN GOWEK. 517 



filled a hollow fossil, its surface against the residual space, originally 

 empty, is flat — in fact, a bedding-plane. 



Those mudstones in which calcite or dolomite is predominant are 

 the most important, because the}* differ conspicuously from rocks 

 formed under standard conditions. It is true that standard lime- 

 stones frequently contain a considerable proportion of calcite-mud, — 

 so far dolomite-mud has not been detected in them ; included with 

 it in such limestones, however, are the remains of open-sea 

 organisms, for example, crinoids, brachiopods, and corals. Lime- 

 stones which are simply hardened calcite-muds, or contain merely 

 the forms enumerated opposite ' L. Shallow-water characters ' in 

 Table III (p. 514), appear to be coufined in marine series to the 

 distinct groups of strata (Modiola phases) under discussion, or, if 

 found among standard deposits, to an occasional occurrence in 

 shallow-water formations. 



For the origin of the calcareous portion of the muds we do not 

 need to look beyond the detrition and disintegration of organisms, 

 both within and without the area of actual deposition, and also, to 

 some extent, of contemporaneously-formed l limestones (oolites, etc. ) 

 and dolomites, fragments of which are found in the interbedded 

 breccias. It is possible, however, that some, or even much, of the 

 mud has been chemically precipitated. But, in any case, the 

 uniform fineness of grain, — which characterizes the intermingled 

 terrigenous material also, — compels a conclusion of much im- 

 portance, namely, that the sedimentation of the mud- 

 stones was exceedingly gentle. 



Conditions of deposition of the Modiola phases. — 

 Hence we see that the notable feature of the Modiola phases repre- 

 sented in Gower is in each an anomalous association of characters. 

 On the one hand, some of the latter indicate deposition in shallow 

 water : in fact, signs of marked current-action, such as the inclusion 

 of contemporaneous fragments, are to be found, in most localities, 

 at some part of each phase. On the other hand, the texture of the 

 rocks affords repeated evidence of the gentlest sedimentation. 



The explanation of the anomaly is to be sought in peculiar con- 

 ditions of deposition. It is conceived that, as the area of deposition 

 of each phase was shallow, and extended far, both parallel and at 

 right angles to the Avonian coast (Table Y, p. 526), under certain 

 circumstances the outer part of the area would protect the inner from 

 waves and strong currents, even in the absence of subaerial barriers 

 along the outer margin, — and of the existence of such barriers, as 

 previously mentioned, we have no evidence. The necessary attendan t 

 circumstances would be that tides were at a minimum — to which 



1 Some of the mud may have been derived in part from older limestones 

 also, but to an extremely small extent, probably, as regards material deposited 

 in Gower; the phase in Cj + Si may, however, owe more of its material to 

 such sources in those parts of the South-Western Province where it rests upon 

 older Avonian limestones unconformablv. 



