532 MR. E. E. L. DIXON AND DR. A. VATJGHAN ON [.Nov. I9II,, 



Y. Interpretation of the Lithological Sequence [E. E. L. D.]» 



(1) Preliminary Remarks. 



Apart from many minor variations in rock-type, a general 

 sequence of facies (Table II, p. 505) is presented by the Avonian 

 of Gower. In the following interpretation of this sequence it has 

 been assumed that those true t oolites — that is, rocks consisting 

 chiefly of typical, concentric and radi all y- crystalline ooliths, — that 

 are widespread have been deposited in shallow water. The 

 assumption is justified by the current-bedding of such rocks and 

 the frequency of fragments of contemporaneously-formed oolite in 

 them, as well as by what is known both of the conditions deter- 

 mining the formation of ooliths 1 and of the situations in which 

 ooliths are found at the present day. The oolites include some of 

 the purest of the Avonian limestones, but the fact that limestones 

 practically free from terrigenous material may be formed in shallow, 

 coastal waters is so well known as to need no emphasis. 



The differences between the three districts (see Table I, facing 

 p. 505) will be discussed later. 



(2) Variations in Depth and other Conditions of Deposition 

 over Gower as a whole at Successive Times ; 

 Conclusions ; Earth-Movements. 



The variation of depth in time is represented diagrammatically 

 by fig. 8 (p. 533). In this there is no true vertical scale ; the depth- 

 line indicates merely periods of extreme shallowness, and, by the 

 direction of its slope, shallowing or deepening. Eor horizontal 

 scale, the duration of a subdivision is arbitrarily and only ap- 

 proximately made proportional to its thickness. 



In the following discussion, on which the figure is based, separate 

 reference will not be made to the depth of the Modiola phases ; in 

 accordance with the views already put forward, they are represented 

 as extremely shallow, but with some interruption in the case of 

 Km and, to a greater extent, in that of the top of S 2 (see Table III, 

 4, p. 514). 



(i) The change from Upper Old Red Sandstone to 

 Km. — This was a change from 'continental' to marine conditions : 

 it is represented in fig. 8 (p. 533) by the depth-line passing below 

 sea-level. The change was sudden, and was unaccompanied, so far 

 as known, by marine intercalations in the top of the Old Eed or 

 continental recurrences in the base of Km ; but, despite a little 

 current-action, deposition was practically continuous. 



(ii) The change from Km to K r — Sharp increase of depth. 



(iii) Conditions during K. 2 — The sea was muddy, but sup- 



1 G. Linck, Neues Jahrb. Beilage-Band xvi (1903) p. 495; and L. W. 

 Collet, ' Les Depots marins ' Paris, 1908, pp. 282-83. 



2 On the supposition that K is equivalent to the Lower Limestone Shales 

 (see p. 496). 



