496 J. Starlde Gardner — Relative Ages of 



the Eocenes, as the conditions of deposition do not seem to have been 

 quite wholly dissimilar. The approach towards Tertiary types, 

 however, is as yet very inappreciable, though for almost the first 

 time a few Gasteropods of fusiform type make their appearance. It 

 is also especially noteworthy that though the plants are in con- 

 siderable variety, they are exclusively gymnospermous. A signifi- 

 cant fact in the elucidation of its deposition is that the Gault 

 basement bed includes on the French side of the Channel a fossil. 

 Ammonites mammillaris, which is wholly confined to the Aptian in 

 England, only about 30 miles distant. 



Succeeding to, and possibly in part contemporaneous with the 

 Gault, is a heterogeneous collection of sands and muds with glau- 

 conitic grains called collectively the "Upper Greensand." The 

 varieties of rock included in it are seldom superimposed, and were 

 probably deposited in different ages and under diiferent physical 

 conditions. They, doubtless, correspond to the Green muds and 

 sands of the " Challenger," and form the base of the true Chalk, 

 into which they often itnperceptibly merge. 



Chalk. — The Chalk is a vast formation which has suggested the 

 name for the whole system, estimated to be 1500 feet thick in the 

 Isle of Wight, end exceeding 1100 feet in Norfolk. It is slightly 

 marly and glauconitic at its base, but quickly passes into a pure 

 white limestone composed of the remains of foraminifera, valves of 

 Cytherina, excessively minute infusoria, cell prisms of Inocerami, 

 sponge spicules and other debris of organic life. The only extraneous 

 substances in it are flint, chalcedony, oxide of manganese in the 

 state of dendritic markings, and oxides and sulphides of iron, all of 

 which have apparently been separated and segregated since its 

 upheaval above the sea-level. It stretches from England, through 

 France, Germany, Poland, and Southern Eussia, to Persia and India, 

 much of this enormous tract retaining the characters with which we 

 are so familiar in England. It is difficult to estimate its original 

 extent, for we have evidence in our own country that it has been 

 completely denuded from the Scilly Isles, Wales, and a large part 

 of Scotland, and our Lower Tertiaries are to a large extent composed 

 of its debris. 



Its vast extent, homogeneous nature, and freedom from terrestrial 

 impurity, are characters which appear extremely difficult to reconcile 

 with any but an oceanic deposition remote from land. It bears in 

 fact the greatest resemblance to Globigerina ooze, while its larger 

 organisms, composed mainly of Echinodermata and Sponges, are, with 

 some exceptions, such as are now met with in abyssal depths. Here, 

 if anywhere, we have a truly oceanic deposit, stretching across the 

 heart of a continent, and the advocates of the permanency of conti- 

 nents have therefore exhausted every argument to demonstrate that 

 it is not an equivalent of Globigerina ooze. These arguments take 

 no account in comparing analyses of freshly-dredged ooze and 

 chalk, that the latter has been elevated for ages, during which it 

 has acted as a sponge for the collection and percolation of rain- 

 water charged with carbonic acid, which has ceaselessly been re- 



