LOWER CHALK, AND CHALK-MARL. 11 



CHLORITIC MARL. 



At the base of the Chalk, and dividing that formation from the Upper Greensaud, is 

 a remarkable fossiliferous bed, full of green specks of silicate of iron, and called, in 

 consequence, Chloritic Marl, which at St. Catherine's Down measures five feet in thick- 

 ness. This Marl is characterized by a suite of fossils, some of which, as Scc/jMtes iequalls, 

 Sow., here appear for the first time, and seem to be special to the bed ; with these are 

 found Ammonites varians. Sow., Amm. sjjieiidens, Sow. ; several Protozoa belonging to 

 the genera Spoiiffia, SipJionia, and Sci/pJda ; Echinodermata, as Ananchjtes lavis, Deluc ; 

 Catopijgus carinafus, Goldf. ; and Biscoidea subuculus, Leske ; together with several species 

 of MoLLUscA. The same stratum occurs near Chardstock, from whence I have obtained 

 many fine specimens of Psciidodiadcma tumidum, Eorbes, P. siibnudim, Ag., Pedinopsis, 

 Holedypm, and several other species, most of which are common to this rock and the 

 Upper Greensand, of which it probably forms the uppermost bed. 



The Upper Greensand appears to correspond to the Glaucoiiie craycuse of the French, 

 the Tourtiu of the Belgians, the Grunsand of the Germans, and the Etagc Cenomanicn of 

 d'Orbigny. 



THE LOWER CHALK, AND CHALK-MARL. 



The Chalk formation occupies a large area in the Isle of Wight, and in the southern 

 and eastern parts of England. It consists of nearly piu-e carbonate of lime, and in many 

 cases is almost entirely composed of microscopic shells, either fractured or entire. My friend 

 H. C. Sorby, Esq., F.G.S., by preparing thin slices of chalk on slides of glass for microscopic 

 examination, has shown that many beds of that rock consist of from 90 to 95 per cent, 

 of the cases of Forcminifera, and of comminuted shells. The chief difference between 

 the Upper or soft white Chalk, and the Lower or hard Chalk is caused by the filling up of 

 the cavities of the shells by calcite or crystalline carbonate of lime, wlierc it has pro- 

 bably been deposited by infiltrating water, which has carried away some of the lime in 

 percolating through the higher beds. The Chalk formation is divided into Chalk-marl at 

 the base. Lower or hard Chalk without flints, and soft or Upper Chalk with flints. In the 

 Isle of Wight the whole formation is 1300 feet in thickness, whilst in England it varies 

 from 600 to 900 feet. 



The Lower Chalk near Dover is of a grayish colour, and much indurated in parts. It 

 is very rich in Echinida, and contains several new^ species. Unfortunately, many of the 

 finest specimens are impregnated with iron, and perish by the decomposition of the 

 pyrites. At Lewes, in Sussex, it is a hard, close-grained rock, with an earthy fracture, 

 and contains many urchins in fine preservation. 



The following section, by the Rev. W. D. Couybeare, of the Clialk cliff's near Dover, 



