48 On the Mirwalogkal Geography 



inquiries were limited, have been deposited in a vast hollow 

 or gulf, the bottom of which was ot chalk. 



This gulf perhaps formed a complete circle, or a kind 

 of great hike ; but we cannot ascertain this, in consequence 

 of its edges on the south-west having been covered, as well 

 as the materials of which they were composed, by the great 

 sandy platform first mentioned. 



We may add that this great sandy platform is not the 

 only one which has covered the chalk ; there are several in 

 Champagne and Picardy, which, although smaller, are of 

 a similar nature, and may have been formed at the same 

 time. Like it, they are placed immediately over the chalk, 

 in the places where the latter was so high as not to admit of 

 its being covered with the materials ot the basin of Paris. 



We shall in the first place describe the chalk, the most 

 ancient of the substances which we have in our environs, 

 and conclude with the sandy platforuj^ the most recent of 

 our geological productions. 



In the intermedium between these two extremes we shall 

 speak of less voluminous but more varied substances, which, 

 had covered the great cavity of the chalk before the platform 

 of sand was deposited on some of them. 



These substances may be divided into two soils (elages). 

 The first (which covers the chalk wherever it was not suf- 

 ficiently hiofh, and which has filled the whole of the bottom 

 of the gulf,) is itself subdivided mio two parts of equal 

 level, and placed not upon one another, but end to end : 

 viz. 



The platform of siliceous lime containing no shells. 

 The plati'orm of lime with coarse shells. 

 We are sufficiently well acquainted with the limits of 

 this soil on the chalky side, because the chalk does not 

 cover it; but these limits are marked in several places by 

 the second s©il, and by the great sandy platform which 

 forms the third, and which covers a great part of the two 

 others. 



The second soil will be named gT/pso-marle7/. 

 It is not generally spread, but merely scattered from 

 space to space, and as it were by spots ; these spots also, 

 are very different from each other in thickness, and in the 

 details of their composition. 



Ttiese two intermediate soils as well as the two extreme 

 soils are covered, and all the vacuities which thcy.hav^e 

 left are partly filled, by a fifth sort of soil, mixed also with 

 niarle andsilex, and which we caWfres/i-water soily because 

 ft abounds in fresh-water sheila only. 



We 



