431 



MINERALOGY. 



Paris build 

 ing stone. 



Geognosy, green sand, already described as lying under the chalk 

 formation, and abounds on marine shells. 



In the superior beds, there are layers several feet thick, 

 of a pale-yellow limestone, which is pretty hard, and in 

 large grains, and forms the principal building-stone at 

 Paris. They contain great abundance of petrifactions, 

 and particularly of cerites, and are covered by marly 

 beds. Siliceous productions occur principally towards 

 the upper part of the formation, and these are flint or 

 flint passing to hornstone, and crystals of quartz. In 

 some places the siliceous matter occurs in considerable 

 masses, even whole beds, sometimes in the form of 

 sandstone, which abound in marine shells, and occa- 

 sionally also contains fresh-water shells, such as lym- 

 naea and cyclostomse. 



3. Siliceous Limestone. 



Above the coarse limestone is a limestone which con- 

 tains fresh-water shells. It is [stratified, and the strata 

 are sometimes soft and white, sometimes grey and com- 

 pact, and penetrated with silica, in all directions. 

 Sometimes the walls of cavities and fissures are lined 

 with calcedony and small crystals of quartz ; in other 

 instances the siliceous matter is formed into masses of 

 vesicular and corroded quartz, named buhr or mill- 

 stone, which has been regarded as the skeleton body 

 of a siliceous limestone, the calcareous portion of which 

 has been removed. 



6. Sand and Sandstone. 



Geognosy. 



Siliceous 

 limestone. 



Buhr 

 etone. 



Gypsum. 



4. Gypsum.' 



The gypsum formation rests on the beds just de- 

 scribed. It consists of an alternation of beds of gyp- 

 sum and marl. Where thickest, as at Montmartre, near 

 Paris, it is divided into three beds. The lowest bed is 

 composed of thin layers of gypsum, which is often la- 



Resting upon the marls when the suecession of beds Santl and 

 is complete, there is a great bed composed of sand 8andstone - 

 and sandstone, sometimes 300 feet thick. 



The sand consists of angular particles of quartz, 

 mixed with earthy carbonate of lime, and fragments of 

 shells ; sometimes it is quite pure and thin ; it is used 

 for making plate glass. There arises from amongst 

 this sand a kind of sandstone, which is used in paving 

 the streets of Paris, and the roads in its vicinity. It 

 is composed of transparent shining angular particles of 

 rock-crystal immediately connected together, or with- 

 out any basis or cement. Sometimes the particles are 

 so loosely aggregated, that they can be separated by 

 the simple pressure of the finger, while in other cases 

 they are so closely aggregated as to pass into a state 

 nearly approaching to that of compact quartz. The 

 sandstone or quartz is disposed in the sand in various 

 ways ; sometimes it is in large or small imbedded 

 masses, or in beds sometimes several feet thick, and 

 extending to a considerable distance. This bed of 

 sand and sandstone contains few marine organic, re- 

 mains, and those that do occur are confined to its upper 

 part, where it is intermixed with a calcareous sand. It 

 is indeed a general observation, that animal remains 

 occur very rarely in quartzy rocks, but abundantly in 

 those of a calcareous nature. 



Sometimes the sandstone contains cotemporaneous 

 portions of flint, homstone, flinty-slate, and thus a con- 

 glomerate is formed. 



7. Fresh Water Limestone, with Millstone, or Buhr- 

 stone. 



There frequently rests immediately upon the pre- Fresh wa- 



--J-- ~ OJ I J \ J ~ J f 



mellar, of solid calcareous marl, and slaty argillaceous ceding sand and sandstone, a thick bed consisting of ter lime- 



Bones of 

 quadru- 

 peds. 



Marl. 



Celestine 

 balls. 



marl, in which menilite is found ; and the under part 

 of this bed sometimes encloses marine shells. In the 

 second bed, the gypsum is in great quantity, but the 

 marl in small quantity ; no shells occur, but petrified 

 fishes are met with. The uppermost bed, which is the 

 principal one, is four times thicker than the others, 

 and is that which is quarried on account of its gyp- 

 sum ; thin beds of marl occur ; the gypsum is some- 

 times 60 feet thick ; it is naturally divided into large 

 irregular prisms like basalt ; it is pure, and is gra- 

 nular foliated ; the lower part often contains flint, 

 and the superior part passes into marl. This bed is 

 particularly distinguished by the multitude of bones, 

 particularly of quadrupeds, which it contains ; and it 

 also contains some fresh water shells. This formation 

 at its line of junction with the rock of limestone, on 

 which it rests, is intermixed with, and passes into 

 it. 



5. Marl. 



The marls that rest upon the gypsum, and which 

 often replace it, are of two kinds ; one is of the same 

 nature as that which alternates with the gypsum, and 

 contains fresh water shells, while the other contains 

 marine shells. 



The first are in general white and calcareous ; they 

 contain silicified trunks of the palm tree, and species of 

 lymneus and planorbis. Above these are beds of argil- 

 laceous marl, which are sometimes 60 feet thick. In 





layers of sand, marl, clay, and millstone. The mill stone, with 



or buhr-stone, is quartz, in a vesicular and corrod- 



ed form, and which forms beds sometimes upwards 



of one hundred feet thick. Its vesicular form adapts 



it for millstones, and hence it is very extensively quar- 



ried for this purpose, and exported to England and 



other countries. 



Above these layers, and forming the uppermost por- 

 tion of the series of rocks which rests upon chalk, is a 

 formation of limestone different from those already de- 

 scribed, and which extends to very considerable dis- 

 tances. It is highly impregnated with quartzose mat- 

 ter, and contains abundance of fossil shells, which are 

 said to belong to the division of land and fresh tvater 

 shells, and hence this limestone has been named. fresh 

 water limestone. It is of a yellowish white colour, 

 fracture earthy or conchoidal, varies in hardness from 

 that of compact limestone to that of the softer marls. 

 It frequently contains flint and hornstone, and also 

 beds of the vesicular quartz, or buhrstone, which, in 

 general, is more compact than that found in the pre- 

 ceding formation. It is further particularly distin- 

 guished by its containing numerous irregular cylin 

 drical cavities, and by the animal remains it contains, 

 resembling, in characters, the genera lymnaea, planor- 

 bis, cyclostoma, and helix of our present marshes. 



Observations on the Paris Formation. 



This remarkable group of rocks has been sometimes Oberva- 



these beds balls of celestine or sulphat of strontites distinguished into four divisions, or beds, according to tjons on the 

 occur. It is succeeded by several thin beds, and the the kind and distribution of the organic remains it^ 

 whole is terminated by two beds, containing vast abun- contains. The Jirst or lowest bed includes the plastic mi 

 dance of oysters. clay, coarse limestone and inferior sandstone, in all of 





