368 



SCIENCE- G OSSIP. 



A HISTORY OF CHALK. 



By Edward A. Martin, F.G.S. 



{lOontinttcd from page 337.) 



'T'HE Upper Greensand has been divided by Dr. 

 Barrois into two zones — viz. the zone of 

 Pecten asper above, and the zone of Ammonites 

 inflatus below. Taken as a whole, it may be said to 

 consist of clays and sandstones, with thin bands of 

 calcareous grit known as Chloritic Marl ; this latter 

 contains an abundance of phosphatic nodules. The 

 Chloritic Marl may be held to represent the pro- 

 duct of the action of an advancing sea. As erosion 

 went on, fossils derived from strata undergoing the 

 process found their way into the Marl forming. 

 According to Murray and Renard ("Deep Sea De- 

 posits," p. 396), phosphatic nodules tend to accumu- 

 late where currents meet, and it is well to bear this 

 in mind in connection with the many nodular layers 

 in the Cretaceous formation. If a shore on which 

 an influ.i of currents played were undergoing depres- 

 sion, the result would be that the layers of nodular 

 Marl forming would represent different phases of time, 

 and as in the case of the Chloritic Marl of the West 

 of England, would contain fossils proper to various 

 zones. It has been estimated that at the shallowest 

 the Marl was laid down in about 100 fathoms of 

 water. 



The Chloritic Marl derives its name from the 

 grains of glauconite scattered throughout the Marl. 

 The particles are of a dark olive-green colour, and 

 are of a crystalline nature. In chemical composi- 

 tion they consist chiefly of silicates of aluminium and 

 magnesium, and usually contain a proportion of iron. 

 This Marl constitutes a basement bed to the chalk. 

 Jukes-Browne says: — "There is evidence of strong 

 current action at this epoch, and these currents seem 

 to have swept away portions of the deposits where 

 they formed the sea-bottom, sifting the soft marls 

 and sands, washing out such fossils as were hardened 

 by the deposition of phosphate of lime, and incor- 

 porating them in the basement-bed of the new 

 formation. In the South of England it contains 

 fossils derived from the Pecten asper zone ; but in 

 Bedford and Cambridge the derived fossils have been 

 obtained from the gault, and the bed is sometimes 

 called the Cambridge Greensand." The Cambridge 

 Greensand is a coprolitic bed, and where it occurs 

 the gault proper is but a few feet thick ; whilst the 

 Greensand is crowded with fossils, both derived and 

 otherwise. Nodules of phosphate of lime are in such 

 abundance as to have been largely worked for arti- 

 ficial manure. 



In a similar way to that in which, as we have 

 already seen, the remains of animals are even now, 

 in our existing seas, being enveloped at certain depths 

 in oxide of manganese, so here, as the sea became 

 shallower, animal remains became phosphatised and 

 were imbedded in the deposit forming ; along with 



boulders from neighbouring shores where the older 

 rocks of slates, schists, and granites were exposed. 



The Cambridge Greensand is remarkable for the 

 evidence that it affords, by its included fragments of 

 various rocks, of the source from which it has been 

 derived. The currents that brought together the 

 derived material of the bed also brought fragments 

 of basalt, granite, felstone, sandstones, quartzite 

 and slate, with an occasional piece of limestone. 

 Mr. Whitaker suggested in his " Geology of London '' 

 that these fragments were derived from a tract of 

 land off our eastern coasts, which, before the Chalk 

 was laid down, formed an exposed area and suffered 

 denudation at the time. Also from the nature of these 

 fragments we may be fairly certain that the then- 

 e.xposed area was one of ancient palaeozoic roclcs. 

 In a boring at Culford, 25 miles from Cambridge, 

 palaeozoic rocks have since actually been found 

 immediately beneath the Gault Clay. It is possible, 

 as Jukes-Brown points out, that the derived nodules 

 of the Cambridge Greensand may have been ob- 

 tained rather from the Gault, which probably exists 

 underground in the neighbourhood of Camljrldge, 

 and nearer to their present home ; whilst on the 

 erosion of the Gault they became transferred to the 

 bed that was forming, and which we now call the 

 Cambridge Greensand. 



While the Gault and the Upper Greensand were 

 being deposited in the east, a local deposit was 

 taking place in Dorsetshire and Devonshire, that has 

 been termed the Blackdown Greensand. A corre- 

 sponding depression to that of the Gault Sea was in 

 existence here, although, perhaps, not to the same 

 extent. This bed is probably of the same age as 

 part of the Gault, and the Upper Greensand, and is 

 deposited unconformably upon Triassic beds. 



The greater part of eastern England must have 

 sunk beneath the waters of the Gault Sea, but unlike 

 the conditions of sea and land areas in the true 

 Chalk age, Wales probably was still dry land, as 

 well as the north-west of England. A change, how- 

 ever, came over the land in tKese parts. The 

 shallowing of the Gault Sea, which altered the 

 character of the deposit from a clay (Gault) to a sand 

 (Upper Greensand), did not continue until the 

 succeeding Greensand was actually raised above the 

 ocean. There is no palaeontological break between 

 the Upper Greensand (Chloritic Series of Lyell) and 

 the early Chalk beds, so we may safely assume that 

 deposition went on continuously. The land, too, 

 after possibly remaining almost stationary for a long 

 period, commenced to sink. The Lower Chalk beds 

 then began to be formed. With these we propose to 

 deal separately. 



(lo be continued.) 



The Jubilee Celebration Meetings of the Royal 

 Meteorological Society were most successful, and 

 included commemoration meeting, conversazione, 

 excursion to the Royal Observatory, Greenwich, and 

 a dinner. 



