THE GLACIAL PERIOD. 221 



been frequently described, and their origin has been a much 

 disputed question. M. de Cossigny has recently renewed the 

 interest of French geologists in the subject, and presented them 

 with a very clear description of the phenomena as displayed in 

 the southern part of the Paris Basin, from which we learn that 

 there are two separate layers of superficial dtbris — the lower one 

 an unstratified clay with flints, and the upper a deposit of sand 

 also containing flints. The lower bed, which rests immediately 

 upon the chalk, consists of a confused mass of flints, the inter- 

 stices between which are filled with a white or yellow clay, 

 which, on account of its refractory properties, is much used for 

 the making of the ovens or kilns in which porcelain is baked. 

 All the flints come from the chalk — not only from the Lower 

 Cretaceous strata which are now all that remain of the forma- 

 tion in the region described by M. de Cossigny, but also from 

 different stages of the upper division, as is proved by the fossils 

 which the flints contain. " These flints," he says, " are quite 

 unaltered ; they have preserved their natural colour, their tex- 

 ture, their white porous surface ; they are only, for the most 

 part, more or less broken, but the fractures are always fresh, 

 the angles perfectly sharp, and when they are washed and 

 divested of the clay that adheres to their surface, a mineralogist 

 could not distinguish between them and the flints recently 

 extracted from one of the chalk -quarries of Normandy. This 

 state of perfect preservation is due without doubt to the im- 

 permeable nature of the clay in which they are imbedded, 

 which has protected them from atmospheric influences. But 

 what is most worthy of notice is the fact that they show 

 no trace of weariog, and have evidently never been rolled 

 about by water." Those of a spheroidal form are frequently 

 entire, while those of irregular shape have nearly always been 

 divested of their projecting knobs and tubercles. Again, of 

 those which are about the size of one's fist, and which have 

 not preserved their original shape, the greater number appear 

 to have been reduced by having a succession of large flakes 

 struck from them, which has caused them to assume a rudely 



