Seeley — Rock of the Cambridge Greensand. 303 



When the workable bed is washed for commerce, water carries 

 away the fine chalky mud ; and then, all that remain are phosphatic 

 nodules, glauconite, and sometimes a few large stones ; so that the 

 name Green sand is a misnomer when this marl is compared with 

 the silicious rock which first gained the name. This variable cha- 

 racter of the sea-bottom in different areas must certainly have in- 

 fluenced the fauna of the region ; and therefore in the comparison of 

 sandstone and limestone, whether of this series or other beds, it will 

 generally be desirable to compare the fossils of the several districts 

 separately rather than those of one area with remains from all the 

 others ; both because the physical conditions will rarely be uniform 

 over a large sea, and because we may in this way track the relation 

 of province to province, and the migration of species consequent on 

 an altering sea-bed, or uplifting or sinking of land. 



There is nothing in the character of the stratum to suggest the 

 time required for its deposition ; or whether it occupied the begin- 

 ning, the close, or the whole of the G-reensand age ; for too much 

 stress must not be laid on its conformability with the Chalk and un- 

 conformability with the Gault, for these terms are only true when 

 used relatively. Yet, from its relations to Upper Greensand, north 

 and south, I regard it as representing the entire period. And while 

 the green colour in a certain way connects the Cambridge bed with 

 its southern extension, its calcareous and phosphatic features connect 

 it with its northern extension, the Eed Eock of Norfolk and the 

 Wolds.i 



In the bed are large stones ; included erratics. They appear to 

 have rarely been much worn, since the angles are often sharp ; and 

 must have laid quietly on the bottom, since they are usually over- 

 grown with Plicatula sigillina and Ostreoe. We have been careful 

 only to preserve specimens with these seals of genuineness on them, 

 as the Greensand is often capped with gravel. None of the indu- 

 bitably genuine specimens show a trace of fossils. 



The blocks vary in size from an inch to a foot ; and include several 

 of granite, some quite rotten; micaceous sandstone with numerous 

 small garnets is not rare ; hornstones and quartzites of various 

 colours and qualities are common ; various metamorphic rocks occur, 

 which have undergone different degrees of change and sometimes 

 show cleavage. There are traps and rolled sandstone. The slaty 

 metamorphic rocks are less worn than the granites. 



The Portland rock may owe its extraneous specimens to the 

 Carboniferous, and other strata to a like cause. 



Now these blocks are the only representatives of the silicious part 

 of the Greensand. They seem to me to be fragments of the -parent 

 crystalline rocks whence the mass of the Greensand of thicker sec- 

 tions came. If the Cambridge Greensand is so thin, it can only be 

 because no material for forming a rock was drifted here; and this 

 appears to have resulted from the whole of this county, and certain 

 parts of Bedfordshire, and the district S.W., having been a stationary 

 sea-shore. Here, then, would naturally be stranded or thrown up 

 1 See Geol. Mag., Vol. II., p. 529. 



