278 M. Peltier on Changes hi Metallic Conductors. 



prove evidently that the macigno formation is wholly inde- 

 pendent of the chalk formation, and that it may be separated 

 from it by characters of a greater value than those which have 

 served to determine the distinction of the Devonian and Silu- 

 rian carboniferous formations. We must consider it as the last 

 secondary deposit, occupying a place between the chalk and 

 the tertiary formations. In the period in which it was depo- 

 sited a change had taken place in the nature of the sediments 

 in relation to those of the anterior (cretaceous) period ; the 

 one had been entirely calcareous, the others in great part 

 arenaceous. In the period of the macigno, the family of the 

 lludist(E had ceased to people the seas of the south of Europe, 

 and with it had disappeared also the Nerinea;, and almost the 

 whole of the ActionellcB ; only some few species of Nummu- 

 lites and Ammonites had continued their languishing existence, 

 to become extinct at the end of these deposits. These consi- 

 derations authorize me then to distinguish the macigno as a 

 formation of a particular age, and to assign to it a special name, 

 on account of the great part it acts in the sediments of southern 

 Europe. I propose to name it the Etrurian System, for the 

 reason that it has been recognised for the first time in a clas- 

 sical manner in the soil of Tuscany. I conclude these obser- 

 vations on the macigno, and on the chalk formation of Italy, 

 by confronting their divisions in a Table with those of the 

 northern chalk. 



Northern zone. 



Wanting < 



White chalk J 



Upper greensand . -j 



Gault 



Lower greensand .... 



Southern zone. 



Etrurian formation .. 



Upper chalk formation 



Lower chalk forma- 

 tion 



Alberese. 

 Macigno. 

 Indistinct. Limestone 



with Ostrea vesicularis, 



Catillus, Belemnites mu- 



cronatus, &c. 

 Glauconie and nummulitic 



limestone. 

 Wanting, or indistinct. 

 Neocomian formation. 



XL. On the Changes produced in Metallic Wires which have 

 served a long time as Electric Conductors. By M. Peltier, 

 in a Letter to M. Arago* . 



nPHE important and beautiful experiment which you are 

 -* about to make on the conductibility of wires, in the 

 establishment of an electric telegraph, induces me to commu- 

 nicate to you two facts which are incidentally connected with 

 this subject. About twelve years ago, being desirous of tracing 

 * From the Comptes Rendus, &c., January 1845. 



