296 J. F. BLAKE AND W. H. HTTDLESTON ON 



The upper of these beds is instructive, as showing how Thamnas- 

 traean and Thecosmilian reefs amalgamate and inosculate; but the 

 main interest attaches to the lower. We have here, indeed, a series 

 of limestones containing Ciclaris florigemma, undoubtedly under- 

 lying the Eag, and not alternating with it, as the similar though 

 false-bedded ones appear to do at Calne. The association of Avicula 

 ovalis points to an earlier age, and would induce us to compare these 

 shelly limestones to the series below the Eag, as its position here 

 indicates, and to note that in this district, at least, the Urchin ap- 

 peared before its usual time, of which we have confirmation else- 

 where, and that the lower beds are beginning again to assume 

 importance under a different aspect. We are, in fact, now entering 

 upon the confines of a region which extends, as will be seen, to 

 Marcham, and perhaps even to Oxford, where the more normal state 

 of things no longer obtains. The thickness of the limestones is 

 seen to be not inconsiderable, and has probably been the cause of 

 the wide extension of Coralliau rocks in the neighbourhood. But 

 little more of them, however, is to be seen, for they everywhere 

 support a luxuriant coral growth, probably the largest reef of the 

 age in England. Every stone is Madreporarian, and the roads are all 

 mended with magnificent specimens of Thamnastrcea and Isastrcza. 



As we approach Highworth, however, the Coral Eag becomes 

 diminished in importance by the development of lower beds at its 

 expense, and is at the same time thrown back from the escarpment 

 to form the surface of the dip- slope of the country near Seven- 

 hampton. Hereabouts small shallow openings, in which not more 

 than 5 feet are seen, seem to bespeak its thinness, and to show, by 

 their position above the brooks that are cut down to the Oxford Clay, 

 that it is developed where the lower beds have also thinned on the 

 dip, the whole of the Corallian beds in the region between Seven- 

 hampton and Watchfield being not more than 30 feet thick. It pre- 

 sents no features of importance as distinguishing it from the similar 

 beds to the west, except some admixture of clay. It is seen to rest 

 sometimes on a semi-oolitic stone, full of calcite, and sometimes on 

 clay, and is also overlain hereabouts in the direction of the dip, which 

 is southerly in the main, by a considerable mass of reddish sands, 

 which at Shrivenham village are of importance ; but of their precise 

 nature and thickness we have found no opportunity of judging. 



Our interest here becomes transferred to the beds below the Eag, 

 which are finely developed in the quarries about Highworth, one of 

 which, the most important, has already been described, first, by Mr. 

 Lonsdale, and, secondly, in the Geological Survey's brief memoir. 

 In these descriptions the measurements and characters of the beds 

 do not differ more than in the quarry itself; but the interpretations 

 given to them are totally at variance with each other. Mr. Lons- 

 dale considers the quarry to represent the whole formation, and 

 states that here the Upper Calcareous Grit can be seen to the greatest 

 advantage ; while in the Survey map it is all coloured Lower Calca- 

 reous Grit, and is said in the memoir to be the " finest section " of 

 these beds. We think the truth lies in the happy mean, and that 



