238 PEOF. S. ir. EEYIS'OLDS ox THE LITHOLOCilCAL [vol. Ixxvii, 



the very constant association of calcareous algae with Modiola- 

 phase conditions. Comparing the Avon development with that 

 shown in Mr. Dixon's table the following points may be noted : — 



First 2Iodiola phase (Km). — Xo conglomerates or oolites occur in the 

 Avon development. Except for the Bryozoa-Bedthe Avon rocks are relatively 

 non- calcareous, and pass gradually into the Old Eed Sandstone below : but 

 they are sharply marked oif from the standard deposits of K^^ above. 



Second Mocliola phase (Co-So). — Thinly-bedded shales and dolomites 

 are more characteristic of Co of the Avon than of Gower. The lower Sj^ Beds 

 form the most typical example of a calcite-mndstone phase in the Avon 

 Section. About 34 feet of strata, mainly consisting of somewhat massive 

 dolomitized limestone with abundant Lithostrotion martini, separate the china- 

 stones and variable rocks of the middle of S^ from the similar rocks at the 

 base of S.,, but it has been thought best to include the latter beds in the same 

 phase as the Co and S^ Beds. 



Third Modiola phase (upper part of S2). — The Concretionary Beds 

 of So form a third Modiola phase, but a good deal of massive limestone 

 is interbedded with the algal limestones and china-stones. The base is 

 ill-defined, the top being more sharply marked by the incoming of the 

 D-limestones with their abundant coral fauna. 



Y. SuilMAET A^D CoNCLrSIOXS. 



Dr. A^aughan's views regarding the phasal conditions nnder which 

 the rocks of the Avon Section accumulated were stated in his report 

 presented at the Winnipeg Meeting of the British Association, 

 while Mr. Dixon, in his work on the lithology of the Gower succes- 

 sion, fulh" describes his conclusions, particularly^ with regard to 

 Jlodio/a phases (calcareous lagoon-phases), and compares those in 

 the Gower area with those of the Bristol area (table v). After 

 a detailed examination of the Avon lithology I find myself so 

 fully in agreement with the views of Dr. Yaughan and Mr. Dixon 

 concernino" the o'eneral conditions under which the Avon rocks 

 accumulated, that it will be unnecessary to repeat these conclusions. 

 The chief points of interest brought out in this paper are : — 



(1) The important part played by calcareous alg^e as rock-builders in the 

 Avon Section. 



(2) The great amount of dolomitization which has affected the Zaplirentis- 

 Beds. 



(3) The predominance of pseudobreccias in the D beds, the fact that a 

 band of this type occurs at the base of K-^, and that some of the pseudobreccia- 

 bands in Do are sandy limestone, instead of, as is usually the case, foramini- 

 feral limestone. 



I am greatly indebted to Mr. E. E. L. Dixon, w^ho has drawn my 

 attention to many points in the field which might otherwise have 

 been overlooked, and has given me the benefit of his opinion on a 

 number of puzzling rock-structures. 



Prof. E. J. Garwood has most kindly examined 1113^ slides of 

 algal rocks, and has lent a number of his own slides for comparison. 

 I have also had the advantage of going over the section with 

 Prof. E. W. Skeats, Dr. Stanley Smith, and Dr. Edward Greenly. 

 I wish to thank mv laboratorv assistant, Mr. J. E. Livingstone, 

 for much willing help in regard to jDhotographic and other work, 

 and Mr. C. J. Bavzand. B.A., for 'white-lettering' Plates XI, 

 XII, & XIII. 



