M. Desnoyers on the Chalk of the Cotentin. 261 



chalk with flint acquire, in two countries distant from each 

 other, (the N.E. of Ireland and the Vicentin,) a compactness 

 with conchoidal fracture, more perfect, more homogeneous 

 perhaps, than any of the Jura limestone beds *, even passing 

 into a granular and spathose structure ; and we are enabled, 

 with the effect, to discover the cause of this alteration from a 

 jirevious state, in the influence of the igneous action of the 

 basalts common to these two countries. 



The uplifting of the chalk at Corfu Castle appears to 

 coincide with its hardness ; as Mr. Webster informs us that 

 this chalk, though exceedingly hard when vertical and con- 

 torted, is soft and tender when horizontal. 



Another circumstance attending the compactness of chalk 

 is its connection with more ancient calcareous deposits, joined 

 to very considerable elevation. Such appear to be those sin- 

 gular deposits of the Alps of Savoy and Bavaria, which have 

 been referred to chalk and greensand, which, notwithstand- 

 ing their great hardness, their sublamellar and compact tex- 

 ture, their black colour, and their elevation above the sea, 

 MM.Brongniart, Buckland, Beudant, Boue, and Deluc agree 

 in referring to this great formation. It was when on the sub- 

 ject of these deposits, that M. Brongniart has shown the little 

 importance of external characters in geology, compared to the 

 value of zoological characters, and relative position. 



The middle beds of the chalk, {glauconie crayeuse, tufau, 

 dialk-marl, &c.) commonly tender and marly in Fi-ance and 

 England, are there in some situations of considerable hard- 

 ness ; and in Prussia, near Quedlingburg, Halberstadt, Goslar, 

 as well as in some other parts of Germany, they approach, as 

 regards this character, the Jura limestone, with which M. Ke- 

 ferstein has sometimes confounded them. The Pianerkalk, 

 which may certainly be referred to the chalk formation, pre- 

 sents the same compactness. Some beds of this formation in 

 the departments of the Basses Alpes, and Alpes Mai-itimes, com- 

 monly possess a no less remarkable hardness. 



The immediate contact of fresh-water limestone on chalk 

 appears also under some circumstances to have hardened it by 

 cementation ; at least I thought I observed this in the environs 

 of Tours and Nogent le llotrou. An analogous spathose 

 ))enetration, most probably, however, independent of fresh- 

 water deposits, appears to have rendered the upper beds of the 

 chalk very compact, and even completely crystalline. MM.Cor- 



• The Jura limestones arc in general much more con)pa(t than their 

 British t(|nival(iit llu- Oolite toniiation.-- Ti-aiis. 



dicr, 



