Lapse of Time between Cretaceous and Eocene Periods. — Table of successive Cretaceous Formations. — Maestricht Beds. — Pisolitic Limestone of France. — Chalk of Faxoe. — Geographical Extent and Origin of the White Chalk. — Chalky Matter now forming in the Bed of the Atlantic. — Marked Difference between the Cretaceous and existing Fauna. — Chalk-flints. — Pot-stones of Horstead. — Vitreous Sponges in the Chalk. — Isolated Blocks of Foreign Rocks in the White Chalk supposed to be ice-borne. — Distinctness of Mineral Character in contemporaneous Rocks of the Cretaceous Epoch. — Fossils of the White Chalk. — Lower White Chalk without Flints. — Chalk Marl and its Fossils. — Chloritic Series or Upper Greensand. — Coprolite Bed near Cambridge. — Fossils of the Chloritic Series. — Gault. — Connection between Upper and Lower Cretaceous Strata. — Blackdown Beds. — Flora of the Upper Cretaceous Period. — Hippurite Limestone. — Cretaceous Rocks in the United States. We have treated in the preceding chapters of the Tertiary or Cainozoic strata, and have next to speak of the Secondary or Mesozoic formations. The uppermost of these last is commonly called the chalk or the cretaceous formation, from creta, the latin name for that remarkable white earthy limestone, which constitutes an upper member of the group in those parts of Europe where it was first studied. The marked discordance in the fossils of the tertiary, as compared with the cretaceous formations, has long induced many geologists to suspect that an indefinite series of ages elapsed between the respective periods of their origin. Measured, indeed, by such a standard, that is to say, by the amount of change in the Fauna and Flora of the earth effected in the interval, the time between the Cretaceous and Eocene may have been as great as that between the Eocene and Recent periods, to the history of which the last seven chapters have been devoted. Several deposits have been met with here and there, in the course of the last half century, of an age intermediate between the white chalk and the plastic clays and sands of the Paris and London districts, monuments which have the same kind of interest to a geologist which certain medieval records excite when we study the history of nations. For both of them throw light on ages of darkness, preceded and followed by others of which the annals are comparatively well-known to us. But these newly-discovered records do not fill up the wide gap, some of them being closely allied to the Eocene, and others to the Cretaceous type, while none appear as yet to possess so distinct and characteristic a fauna as may entitle them to hold an independent place in the great chronological series.