xin. PORTLAND ROCK. 327 



having the cranial and jaw-bones in connection, with cervical 

 vertebrae adjacent. 



The explanation once suggested was that these spheroidal masses 

 Were boulders (of an older part of the Portland series for example), 

 but there is no reason to adopt such a notion. Admitting them to 

 be concretions, round organic masses, there would be some dif- 

 ficulty in accounting for so many heaps of shells &c. at so many de- 

 tached points, but that the same shells occur, though less plentifully, 

 in the intervening sands. Some boulders are formed round bones 

 chiefly. One way of viewing them is to regard them as held 

 together by carbonate of lime, derived from decomposition of ad- 

 jacent shells, or infiltrated from oolitic rocks above. Of these, in 

 Shotover there is now barely a trace left, but there was formerly 

 a small quarry of them, and I believe my memory assures me 

 of lime-burning there half a century ago. It may be regarded 

 as somewhat of a parallel case to what has already been mentioned 

 under the head of the coralline oolite and calcareous grit, and it 

 leads to the same conclusion, of great waste of the oolite in very 

 ancient time, possibly at the conclusion of the long oolitic period. 

 Thus, by meteoric following marine action, the patchy character 

 and local incompleteness of the Portland oolite may be explained, 

 and at the same time the unconformity of the greensand system, 

 which is so remarkable in this region, accounted for. - 



Much in the same way, under the calcareous Portland rock at 

 Swindon, lies an aggregate of sand and spheroidal or irregular, 

 usually flattened, separate sandstones, which lie in planes not quite 

 in conformity with the bedded rocks above. Similar appearances 

 of concretionary sandstones are indeed almost never quite absent 

 from any sections of the Portland sands about Thame, Aylesbury, 

 and Brill, but the spheroids are rarely so large as in Shotover 

 Hill. 



Godwin Austen examined with care the sections presented at 

 Swindon, and in the great quarry at the north-eastern extremity 

 observed the following succession f : 



7. Thin-bedded calcareous sandstones, with marine shells, in a mass of sand. 

 6. Limestone containing apparently only fresh-water shells. 



These parts of the section may be regarded as ' Purbeck beds.' 



-'- - { Geol. Proe. 1850, p. 464. 



