220 REV. J. F. BLAKE ON THE 



that we may match these beds with some part of those at Swindon, 

 and that we have in them only a portion of the series elsewhere found. 



We have seen, indeed, that the Swindon rocks are part only of 

 those at Tisbnry ; but then the former are exceptional in other 

 respects, and higher rocks may have been denuded there, which 

 should be found again further north. There is therefore no par- 

 ticular presumption in favour of the latter interpretation, nor is there 

 any against it ; for though the Portland Sands below are seen to thin 

 out, if we compare Portland, Swindon, Shotover, and Brill, the 

 thinness of them at Tisbury, and perhaps also at Bourton, shows that 

 this is not a general phenomenon. The point must therefore be de- 

 cided by observation of detail. 



If now we compare the northern sections with that at Swindon, 

 and especially that part which is seen on the road to Coate, as de- 

 tailed on p. 209, there is a certain amount of agreement — more than 

 can be found by a comparison with any other part of the series. To- 

 wards the lower part the basal sands often consist of a kind of 

 shelly brash, which will compare with No. 1. The two Trigonia- 

 beds with the white chalky rock with Cardia recall well the three 

 parts of No. 2, especially the Trigonia-hed at the base, which is seen 

 both in Oxfordshire and, in a slightly different form, at Bourton. 

 The glauconitic limestones below these, which are really much more 

 sandy than those above, may well represent, in a changed aspect, 

 the underlying sands, with their preceding rubble beds ; or it may 

 be that the clay of Swindon represents these. If we take this cor- 

 relation and test it by the fossils, we shall find so remarkable a con- 

 firmation that no doubt is left in my mind as to its correctnesss. 

 The following are found in both sets of deposits : — 



Fossils common to the Limestones of Buckinghamshire and the Trigonia- 

 heds of Siuindon and Bourton, and jpecidiar to these or most common 

 in them. 



Ammonites pectinatus {Ph.). 

 ■^Natica elegans (Sow.). 



Trigonia Voltzii (Ag.). 

 ■'^Cyprina elongata {Blake). 

 *Cypricardia costifera {Blake). 

 ^Anisocardia pulchella {Be Lor.). 



*Pleuromya tellina {Ag.). 



Thracia tenera {Ag.). 



Mytilus unguiculatus {Ph.). 



boloniensis {De Lor.). 



^Echinobrissus Brodiei {Wr.). 



Of these such as are marked with an asterisk occur in the creamy 

 limestones, which are thus not to be separated as a higher part than 

 that at Swindon. Other fossils have the same tendency. Thus, the 

 Ammonites are those of low horizons, as are Trigonia Pellati, 

 Lima rustica, Exogyra hruntrutana, and an abundance of Ostrea esc- 

 jpansa, and Pleuromya tellina. 



I regard, therefore, the whole of the limestones of Buckingham- 

 shire as an expansion of the Trigonia-heds of Swindon, except that 

 the brashy beds at the top correspond to part of the " basal sands " 

 of the latter place, but not to the whole. 



If this be the true correlation of the Portland limestones, it follows 

 that, as the freshwater Purbecks succeed them without interruption, 



