Environs of Lons le Saunter. 147 



grains are found in the Jura, which are extremely difficult 

 to be distinguished at first sight from primitive granular 

 limestone ; but in the fracture traces of compact portions 

 occur mixed with the crystalline parts. 



The compact portions commonly form the base of the 

 rock, and the lamellar grains are only subordinate. 



In the oolitic limestone, the size of the grains varies from 

 one bed to the other, from that of a rape-seed to a nut ; but 

 they are nearly all of the same size in the same bed. 



I should observe, that, from the form of its grains this 

 rock was more nearly allied to pisolite than oolite, but as 

 the grains generally are very small; the first denomination 

 would be improper in the greater number of cases. 



However small the grains may be, concentric layers are 

 almost always seen in them round a nucleus, often crystal- 

 line ; this nucleus is sometimes the fragment of a baculite, 

 entrochus, or some other marine shell : now these organic 

 bodies could not have been lapidified and enveloped by 

 stony layers before the formation of the rock ; on the other 

 hand, whatever may be the nature of the other nuclei, the 

 grains enveloping them have probably been formed in the 

 same manner as the others. I conceive then I can with 

 some certainty advance, that in the Jura oolites, the grains 

 did not exist before the formation of the paste, and that 

 this rock has not been formed in the manner of conglome- 

 rates. 



The oolitic and granular limestones alternate, and pass into 

 one another by insensible gradations. 



It contains various proportions of silex ; numerous veins 

 and irregular nodules of black and white flint (silex) occur 

 in it, which, by long exposure to the air, are often trans- 

 formed into spongiform quartz. 



In numerous beds, and on the surfaces slightly corroded 

 by the atmosphere, an innumerable quantity of debris of 

 very minute marine bodies are seen, such as entrochi, ortho- 

 ceratites, baculites, vermiculas, ramified polypi, sponges, 

 &c. These fossils are so much identified with their matrix, 



K 2 



