IS 13.] On Formatims, 199 



with a bed of yellowish argillaceous marl^ which is filled with 

 fragments of sea shells of the genera cerita, trochus, mactra, 

 venus, cardium, &c. ; also fragments of the palate of two 

 species of ray. The beds of marl that succeed these contain 

 principally bivalve marine shells : and in the latter beds, those 

 immediately under the argillaceous sand, contain two beds of 

 oysters. These oysters appear to bave lived in the place where 

 we now find them, for they are united togetlier as in the sea ; 

 the greater number are quite entire; and if we extract them with 

 care, we find that the greater number have both valves. The 

 gypsum formation is often terminated by a mass, more or less 

 thick, of argillaceous sand without sh.ells. 



7. Sand and sandsto?ie w'hIwul shells. — The sandstone without 

 shells is one of the last formations. It constantly covers those 

 already described. 



8. Upper marine sand and sandstone. — This i? termed the last 

 marine formation of the series, and covers the preceding rocks. 

 The sandstone /aries in colour, being sometimes grey, sometinics 

 red. It contains marine shells ; and these are sometimes diffe- 

 rent from those of the lower marine formation. It tlius appears 

 that there are in the vicinity of Paris three sorts of sandstone, 

 sometimes very similar to each other in mineralogical character.*?, 

 but differing in their geognostical position. Tb.e first, or lowest, 

 makes part of the beds in the coarse or marine limestone, and 

 contains marine shells : the second rests on the gypsum forma- 

 tion; and even the marine marl that covers it is the most 

 extensive, but contains no shells: and the third is only covered 

 by what is termed tlie last fresh water formation, and immediately 

 follows the second. It is the least frequent of the three, and like 

 the first contains many marine shells. 



9. Mill or huhrstone formation witliout shells. — This forma- 

 tion consists of feruginous argiiijceous sand, clay marl, and 

 millstone. Thus these substances do not appear to follow any 

 determinate order in their supei position. The millstone is a 

 quartz, containing numerous irregular cavities that do not 

 communicate with one another, and which are traversed by 

 siliceous threads, disposed somewhat like the reticulated struc- 

 ture of a bone, and lined with a crust of red ochre. These 

 cavities are sometimes filled with clay marl, or sand: they are 

 never lined v^-ith siliceous incrustations, like calcedony, nor with 

 crystals of quartz. These last characters, independently of its- 

 position, are sufficient to distinguish it from the millstone beds 

 of the siliceous limestone formation already mentioned. An- 

 other geognostic character of this rock is the want of petri- 

 factions. ^ 



10. Second fresh water form at mn.-~Th\s formation rests on 

 that last described, and is composed of two kinds of rock, the 



