326 THE PORTLAND OOLITE PERIOD. CHAP. 



PORTLAND OOLITE AND SANDS. 



In the Isle of Purbeck and the Isle of Portland are the greatest 

 masses of this rock known in England, and they yield stone of the 

 best quality for building. In the Vale of Wardour in Wilts, at 

 Chicksgrove, limestone of the same age, not oolitic, is found : other 

 detached masses at and near Swindon carry on the interrupted line of 

 deposits, which reappears in Shotover Hill, at Garsington, Great 

 Hazeley, Thame, and Quainton, and culminates in the prominent 

 summit of Brill. These now widely-separated masses may have 

 been once united ; there is much diversity in the appearance of the 

 rocks and their relation to the sands, but a great agreement in 

 the organic contents. The mineral diversity may be explained, and 

 the organic affinity accounted for, by local differences of limiting 

 lands, and a general agreement of connecting water. 



This water was marine, exclusively so, as it appears, for the sands 

 below and above, as well as the rock in the midst. But there was 

 some drift from the land to bring wood in fragments among the 

 cardia, pholadomyse, nerinese, &c. of the Portland limestone and sand 

 of Shotover Hill. This wood was attacked by xylophagous mol- 

 lusks. 



The marine condition here affirmed was everywhere quietly 

 followed by an invasion of estuarine or even lacustrine formations. 

 Thus in Purbeck, the Vale of Wardour, at Swindon, near Shotover, 

 and near Aylesbury, and Brill, pale calcareous deposits with shells 

 of fresh-water affinity followed without disturbance, in parallel 

 strata, as if the area had been gently raised and the sea removed. 

 To this point we must recur in the next chapter. 



In the ascent of Shotover Hill the Portland rock is mainly re- 

 presented by sands of a glauconitic character, enclosing hard sphe- 

 roidal masses in a slight degree calcareous, and mostly very full 

 of shells. They lie in two bands, as well as can now be seen, or 

 rather as formerly could be seen, for many are destroyed. The 

 masses are as much as five feet across and three feet deep. The 

 shells are much intermingled, but offer no special marks of drifting ; 

 some bivalves have the parts in due place ; wood occurs with them 

 penetrated by xylophaga ; there are occasionally bones, one reptile 



