24 



OffiOLOOY OV THE AVON DASTN. 



Mr. TT. M. Smith lias boon f>;no(l cnongli tomnJco for mo, in 

 i'lrjCossor llamsay's laboratory at the University CoIl(!f,''(}, 

 Bristol, chemical analyses of two specimens of this rock, 

 (1) from the Avon Section, (2) from the large quarry near 

 the landing-stage at Portishcad. 



Matter soluble in dilute acid, mostly calcium carbonate 

 ryilica 



Matter insoluble 



vForrio Oxide 



1. 



2. 



00-<l(). 



(;r-08 



!)•;!!. 



r,-Tfi 



24-2.'f. 



2(r7(; 



100-00 lOO'OO 



Microscopic examination of the insolnblo rosidiio Kconis 

 to indicate that the Silica is free as Quartz, and that the 

 Mici'ozoa have been converted into Peroxide of Iron. 



Before passing on to consider the flexures and faults of 

 which wo have evidence at Portishead, it will bo well to 

 summarise the physical features oE the district. This is 

 readily done. To the S.W. "West Hill and Weston Wood 

 form, respectively, the terminations of tlio northorn and 

 southern crests of the Portishead-Cliivodon ridge. Of the 

 Limestone crest, Fore Hill appears to be an outlying portion, 

 separated by a valley partly occupied by Trias. But per- 

 haps the general trend of the southern ridge would lead 

 one to expect that this outlier should be farther south. Of 

 the Old Rod Sandstone crest. Wood Hill appears to be an 

 analogous outlier, separated by a low gap, occupied by TriM,ssic 

 beds. Farther north, instead of anything like a continuatitm 

 of these longitudinal crests, we have a ridge which lies 

 obliquely to them, coinposed of Mountain Limestone, and 

 overlooking the Severn. 



A summary of the geological structure will find a place 

 moi'e litLy at the beginning oC the next section. 



