144 NEOZOIC TIME. [CHAP. IX. 



thick) in the upper part, the highest "beds being light- 

 coloured sandstones without any fossils. Near the Ord of 

 Caithness thick beds of breccia occur amongst the shales of 

 the Kimeridge Clay, and their formation is explained by 

 Professor Judd as follows : 1 " The alternation of the 

 brecciated beds with the finely laminated and quietly de- 

 posited strata and the confused arrangement of the blocks 

 in the former, their admixture with trunks of trees, stems 

 of cycads, and other plant remains, seem to indicate that 

 the quiet deposition of the semi-estuarine beds was inter- 

 rupted by the occasional occurrence, in the rivers just 

 alluded to, of floods of the most violent character. These 

 appear to have swept angular masses, just separated from 

 their parent rock by frosts or landslips, subangular masses 

 which had lain for a time in the course of the streams, and 

 the rounded pebbles of the river-beds, along with trunks of 

 trees torn from their banks, all in wild confusion out to sea, 

 where they were mingled with the sea-derived materials of 

 the shell-banks and shoals." 



The Purbeck Beds exist only in the south of England ; 

 they consist of limestones, shales, marls, and black earths 

 in thin layers, which exhibit alternations of terrestrial, 

 freshwater, and marine conditions. Near Swanage, in 

 Dorset, they are 400 feet thick, but they thin rapidly both 

 to the west and north. In the Vale of Wardour only the 

 lower and middle portions remain, and these are only 70 

 feet thick as compared with 300 feet at Swanage ; at this 

 rate of thinning they would not extend more than ten or 

 twelve miles farther north beneath the Cretaceous rocks, 

 but there is great uncertainty with regard to their northern 

 limit. The freshwater beds of Swindon have been called 

 Purbeck, but it is very doubtful whether these were ever 

 actually continuous with those of the Yale of Wardour. 



1 "Quart. Journ. Geol. Soc.," vol. xxix. p. 195. 



