Mr. Prestwich on the Boring through the Chalk at Harwich. 155 



palaeozoic rocks of the Franco-Belgian area beneath portions of the 

 cretaceous deposits of the South-east of England. Mr. Austen con- 

 siders that this palaeozoic ridge may be partly composed of some of the 

 coal-bearing strata, and that these may be covered in some parts of the 

 South-east of England by little more than the cretaceous deposits; the 

 Lower Greensand having been probably at some points the littoral or 

 shoal deposits of the chalk-sea along this old ridge. Mr. Prestwich 

 expressed his belief that the usual uniform extension of the Lower 

 Greensand was certainly interrupted beneath London, and therefore 

 presented a difficulty in the way of obtaining everywhere the supply of 

 water from this formation which he had once anticipated; still how- 

 ever he thought the interruption in the deposit was but local, as the 

 outcrop of the Lower Greensand to the north, west, and south of 

 London was very uniform in its features. 



" On a Granitic Boulder out of the Chalk of Croydon, and on the 

 Extraneous Rock-fragments found in the Chalk." By R. Godwin- 

 Austen, Esq., F.R.S., F.G.S. 



The boulder which, together with some associated fragments and 

 sand, formed the subject of this communication, was found by the 

 workmen in a chalk-pit at Purley, about two miles south of Croydon. 

 Mr. Symonds drew the attention of Dr. Forbes Young to this inter- 

 esting discovery, and the latter gentleman secured what remained of 

 the boulder after it had been much broken up, and presented it to the 

 Society. The largest remaining fragment is apparently one end of 

 an irregularly oval well-rounded boulder, originally about 3 feet long. 

 The boulder was accompanied by some decomposing fragments of a 

 felspathic trap-rock, and with a compact mass of siliceous sand, 

 which Mr. Godwin-Austen carefully exposed on a visit to the chalk- 

 pit. This collocated mass of rock-fragments and sand the author 

 regarded as being truly Avater-worn beach-material, derived from 

 some old coast-line of crystalline rocks. Other smaller specimens of 

 crystalline rocks, quartzites, &c. have been found in the Chalk of the 

 South-east of England. These are all water-worn : some are quite 

 rounded ; and many of them bear the remains of attached shells and 

 zoophytes : but they are nearly always isolated in position, except 

 at Houghton (Sussex), where they were met with scattered over a 

 uniform level. 



The author proceeded to describe the conditions of the " marginal 

 sea-belt," — where pebbles are found in existing seas, and where also 

 certain molluscs and zoophytes having habits of attachment occur. 

 From such a marginal zone floating sea-weed might have borne the 

 majority of the extraneous pebbles now found in the Ciialk. Of this 

 formation, the author observed that the " White Chalk" ranged as 

 far nortii as a line reaching from the North of Ireland to Riga, on 

 the Baltic, and extended in a broad zone over North Germany. In 

 North Europe tiie conditions of the deposit were very uniform over 

 the Anglo-French area, where 80(J feet is its average thickness, and 

 where it is of deep-water origin. Its fauna, however, is somewhat 

 anomalous ; much of it has drifted from shallower zones of the 



