484 ME. E. E. L. DIXON AND DR. A. VATJGHAN ON [Nov. I9I I,. 



Limits. — The boundary between the two groups is merely the 

 somewhat uneven level at which the dolomitization characteristic 

 of the lower shows greatest diminution when followed upwards. 

 Unevenness of the level on a large scale would explain the 

 inconstancy in thickness of the Caninia Oolite, should it be 

 found that the total thickness of C x is fairly constant. 



The top of the Caninia Oolite is sharply defined, and appears, 

 from the character of fragments in the overlying bed, to have 

 been slightly eroded, in places at least, prior to the deposition 

 of C.„ though it is not channelled or corroded as at West 

 Williamston (Pembrokeshire). 1 



Typical exposures. — ThreeclifT Bay, east side. Caswell Bay, 

 east side : — several outcrops due to folding and faulting. This 

 disturbance, which amounts to an anticline the crest of which 

 has sagged dow^n into a deep syncline with an overthrust from 

 the north for its northern limb, has been described and figured 

 by Dr. A. Strahan. 2 The ' thick bed of light-coloured oolite ' 

 referred to by him is the Caninia Oolite, and the ' dark and 

 thin-bedded limestones ' beneath are the Laminosa Dolomites 

 together, possibly, with the top of Z 2 . Thus the Caninia Oolite 

 has three complete outcrops and, in addition, a partial outcrop 

 due to a strike-fault in the southern limb of the anticline. 

 Longland Bay, north side: — the Laminosa Dolomites with, 

 probably, the top of Z 2 ; in and near Rothers Tor, the Caninia 

 Oolite. 



Upper Avonian. 

 C 2 = Upper Subzooe of the Syeingotrtris Zone. 3 



2. Standard limestones. 

 1. Modiola phase. 4 



Lithological characters. — Of the beds in descending order: — 



2. Standard limestones, that is, limestones with a standard 

 marine fauna, largely crinoids, corals, and brachiopods. The 

 limestones are chiefly light-grey, rather thickly-bedded, and 

 highly fossiliferous, but are in some cases dark and more thinly- 

 bedded, especially those containing many gasteropods. Beds 

 of dark, finely-crystalline dolomite and partings of shaly 

 limestone occur, but are quite subordinate. In the gasteropod- 

 beds the gasteropods are replaced in many cases by dolomite,, 

 even where the surrounding matrix has remained quite un- 

 altered. Selective dolomitization of this kind is found only 

 in organisms which have been originally aragonitic, such 



1 ' The Country around Haverfordwest,' Mem. Geol. Surv. (in the press). 



2 Swansea Memoir, p. 8, fig. 1. 



3 As explained in § VI, the dividing-line between Lower and Upper Avonian 

 is now taken at the base of 2 . 



4 The meaning of this term is explained on pp. 512 et seqq. 



