iSlS.] 



On Format'Iofis, 



shells ; 10. Second fresh water formation of marl and millstone | 

 11. Alluvial. 



1. Chalk. — The chalk, which is the oldest and lowest member 

 of the series, contains a considerable number of petrifactions : 

 of these the most characteristic is the belemnite i 11 species of 

 petrifactions are enumerated, but not one of these has been 

 discovered in the superincumbent formations. 



2. Plastic clay. — This clay varies in purity : the lower bed is 

 the purest, and contains no petritactions : the upper bed, that 

 next the limestone, is sandy, of a blackish colour, and sometimes 

 contains organic remains. It varies in thickness, from 1 7 yards 

 to a few inches. It is distinctly separated from the chalk. There 

 is no transition from the one into the other; on the contrary, thff 

 clay contains fragments of the ch;ilk, a proof that the chalk 

 must have been consolidated before the clay. 



3. Coarse limestone^ and its marine^ shell sandsio?ie.—Thh 

 rock rests on the clay; but it does not every where immediately 

 rest on it, bein^ sometimes separated by a bed of sand, varying 

 in thickness. The lower bed of limestone is mixed v. ith sand ; 

 sometirnes, indeed, contains more sand than lime. This lime- 



^ stone formation is composed of alternate beds of coarse lime- 

 stone more or less pure, clay m.arl, very thin slaty close marl, 

 and calcareous marl, all Jirranged in a regular and determinate 

 order. It is filled with petrifactions : of these upwards of 600 

 have been already described by La Marsk and others. The 

 lowest beds contain petrifactions that do not occur in the middle 

 beds : and in the middle beds v.'e meet with petrifactions that 

 are wanting in the upper beds. It also appears that the number 

 of petrifactions diminish as we approach to the uppermost beds, 

 when they entirely disappear. The middle and upper beds of 

 limestone contain beds of sandstone and hornstone filled with 

 marine shells ; and the sandstone sometimes contains both fresh 

 water and marine shells. 



4. Siliceous limestone. — This formation is composed of slTata 

 of limestone, penetrated with silica. It is often cavernous, and 

 the cavities are lined in sc^:ne instances vvith siliceous stalactites 

 and crystals of quartz. It contains no organic remains. In this 

 formation occurs one variety of the mineral called Im/mlone, 

 used for millstones. The authors of the essay are of opinion 

 that the buhrstone is the siliceous skeleton of a limestone: the 

 quartz being deprived by some unknown cause of its lime, there 

 remains now a porous mass, very hard, and containing in its ca- 

 vities a clay marl. 



5 & 6. Gypsum of^ the first fresh wafer formation^ and marine 

 marl. — The gypsum rests on the siliceous limestone. The 

 formation, however, is not entirely gypseous; but consists Df 

 alternate beds of that mineral and of calcareous and» argillaceous 

 p^rls. We have an excellent example of this formation at 



