CRETACEOUS SYSTEM 



territories. They also occupy wide tract* in the 

 Cult states, whence they extend up the Mississippi 

 valley to tin- Ohio; they put in an appearance at 

 Intervale <>n tin; Atlantic borders between South 

 Carolina ami New .lei>e\ ; and we meet with thrni 

 ajjain on tin- Pacific lxrder and in the coa-st-range. 

 Si rata of the same age occur also in the far west of 

 British America, at the mouth of the Mackenzie 

 Ki\er, and in Crccnland. In India the system is 

 marked in the Deccan by a massive serif* of basal t- 

 r.H-U, 400() to 0000 feet thick, and covering an 

 area of 200,000 sq. in. In Australia and New 

 Zealand there is a considerable development of 

 cretaceous rocks, such as the ' desert sandstones ' 

 of (Queensland, and a small coal-bearing group of 

 beds. In New Zealand the system likewise contains 

 coals, some of which are lignites, while others are 

 bituminous coals of fair quality. The following is 

 the succession of cretaceous strata in England : 



Upper Cretaceous 



Upper Chalk 800 feet 



Middle Chalk 200 .1 



Lower Chalk and Chalk Marl 450 u 



Oiloritic Marl and Upper Greensand 260 



Qault 200 ii 



Lower Cretaceous- 

 Lower Greensand 650 u 



Wealden Series 1860 



The Wealden Beds consist largely of clay and 

 sand, and are almost entirely of fresh-water origin. 

 In Yorkshire, however, the strata which occur on 

 the same horizon as the Wealden beds of the south 

 are of marine origin, as seen in the Speeton clay 

 near Bridlington. The Lower Greensand, consisting 

 of sand, clay, &c., are marine. The Gault (q.v.), a 

 tough blue clay, is likewise marine, and so also are 

 the shallow-water sands of the Upper Greensand, 

 and the thin layer of chalky marl called Chloritic 

 Marl (q.v.), which is characterised by the presence 

 of glauconitic grains and phosphatic nodules. The 

 most characteristic rocks of the system, however, 

 are the chalk beds. The basement of these beds 

 is the argillaceous chalk known as Chalk Marl. 

 The lower chalk is a grayish-white chalk, while 

 the middle chalk is a pure white chalk, containing 

 in its upper portions layers of flint nodules. A 

 hard layer of yellowish limestone called Chalk-rock 

 lies at the top of this division. The upper white 

 chalk is a thick massive white chalk, containing 

 numerous layers of nodules and occasional tabular 

 sheets of flint. All these chalks are of marine 

 origin. The following are the divisions adopted by 

 continental geologists, and now largely used by 

 geologists in Britain : 



Continent*! Cretaceous. Equivalent Engllah Strata. 



Danian (wanting). 



Senonian Uppr Chalk. 



Turonian Middle Chalk. 



/Lower Chalk and Chalk Marl. 

 Cenomanian < Chloritic Marl. 



I. Upper Greensand. 



Albian Gault. 



~ ( Lower Greensand. 



Neocomian | Wealden. 



The cretaceous strata of Britain being almost 

 exclusively of marine origin, it is not surprising 

 that land-plants seldom occur, and that they are 

 met with chiefly in the fresh-water beds near the 

 base of the system. They consist chiefly of ferns, 

 cycads, and conifers a flora resembling that of the 

 preceding Jurassic period. The upper cretaceous 

 rocks of Germany, however, have furnished many 



Elant remains. Amongst those are the oldest 

 nown dicotyledons, such as extinct species of 

 maple, oak, walnut, beach, laurel, magnolia, iV<-.. 

 also several proteaceous plants. A similar admix- 

 ture of forms occurs in the cretaceous strata of 

 North America. Amongst animals the Protozoa 

 played a very important part the white chalks 



and earthy limestones being yerv largely composed 

 of the minute hhell* of foraminiiera, such an Globi- 

 gerina, Kotalia, and Textuluria, which still swarm 

 in tin- oo/e of the Atlantic. Sponges, such as 

 VentriculiU-s, Siphonia, \-c., were very abundant, 

 and sea-urchins also occurred in great numbers. 

 Star-fishes and bryozoans were fairlv common, 

 as were also, amongst brachiopods, TerebratuUi 

 and Rhynchonella. But the orachiopods as a 

 class were feebly represented as compared with 

 their abundance in the earlier stages of the 

 world's history. Ordinary bivalves, however, were 

 very numerous, such as Inoceramus, Hippurites, 

 Spondylus, Lima, Pecten, & f c. In the Danian 

 beds carnivorous gasteropoda begin to abound, 

 and they include a numW of existing genera. 

 Cephalopods are not only the most abundant, 

 but also the most characteristic fossils of the 

 cretaceous rocks. Amongst them are a great 

 variety of Ammonites, and many forms of Belem- 

 nitidii*. Amongst the fishes were ganoids, and 

 various kinds of the shark tribe, together with the 

 earliest representatives of the Teleostei which 

 include most living genera of fishes. The waters of 

 the period seem also to have swarmed with reptiles, 

 such as Ichthyosaurus and Plesiosaurus. \\ inged 

 reptiles were also present, such as Pterodactylus. 

 Amongst dinosaurs were Cetiosaurus, Megalo- 

 saurus, and Iguanodon. Another remarkable 

 reptile was the serpent-like Mosasaurus. Besides 

 these, there were lizards, chelonians, and croco- 

 diles. The American cretaceous system is likewise 

 characterised by the presence of huge dinosaurs 

 and other reptiles some of them being European 

 types, while others are peculiar. One of the most 

 remarkable features of the American rocks, how- 

 ever, is the occurrence in them of the toothed 

 birds Ichthyornis and Hesperornis. (Separate 

 articles deal with most of these various forms. ) 



No break separates the Jurassic from the creta- 

 ceous system there is a gradual passage from the 

 upper beds of the former into the lower beds of the 

 latter. At the beginning of cretaceous times mosfc 

 of the British and Irish area existed as dry land. 

 Over the south-east of England lay the estuary of a 

 large river, flowing probably from the north. The 

 Wealden beds are the delta-deposits of that river ; 

 the English and French beds of this division cover- 

 ing an area of 20,000 sq. m. The sea into which 

 that river flowed occupied a considerable area in 

 the north of France, spread over the Low Countries 

 into Hanover, filled the basin of the North Sea, 

 and overflowed a portion of eastern England. 

 Wealden beds occur in north-west Germany, and 

 indicate the delta of a river, like that of the British 

 area, flowing from the north. While land-condi- 

 tions predominated in northern and middle Europe, 

 an open sea covered vast areas in southern Europe. 

 Gradual subsidence of the sea-l>ottom took place 

 during the deposition of the Wealden series, and 

 eventually the great deltas became submerged, and 

 a wide sea covered most of what are now the low 

 grounds of the British area, and passing eastwards, 

 submerged vast regions of middle Europe up to the 

 slopes of the Ural Mountains. The depression was 

 greatest in the western areas, where in the deep 

 clear waters there gradually accumulated the cal- 

 careous matter which subsequently formed our 

 white chalk. There is no deposit forming at 

 present which is quite analogous to white chalk. 

 The calcareous oozes of existing seas which most 

 resemble it are of abyssal origin, but the sea in 

 which the chalk accumulated probably did not 

 exceed 1000 or 2000 feet in depth. The extreme 

 purity of the chalk, consisting as that rock does of 

 95 per cent, and more of carbonate of lime, is difli- 

 cult to account for on the supposition that the sea 

 in which it formed was comparatively shallow. 



