274< Prof. L. Pilla on the Etrurian St/stem, 



which accompany it, have caused doubts in my mind respect- 

 ing the position which is generally assigned to it; I would 

 even say, have convinced me that this formation is wholly di- 

 stinct, and independent of the chalk. I have been led to this 

 conclusion by the exposition and the analysis of the facts 

 which will form the object of the memoir which I have the 

 honour to submit to the Academy. I shall divide this me- 

 moir into three parts : — 1st, the distinction of the chalk forma- 

 tion in Italy ; 2nd, a comparison of the chalk formation of 

 Italy with that of France and of England ; 3rd, the indepen- 

 dence of the macigno and its geological position. 



1 . Distinction of the Chalk Formation in Italy. 



It is generally agreed to divide the chalk formation in Italy 

 into two portions, the upper and lower. The first is composed 

 of the macigno ', the second of a limestone, which, being cha- 

 racterized principally by species of Rudista, may be called 

 hippuritic. 



The macigno formation is so well known, that I shall not 

 dwell long in describing its characters. I shall only observe 

 that it is composed of two kinds of rocks, of a calcareous marl 

 alternating with schists, which is commonly named Alberese in 

 Tuscany, and of the macigno properly so called. The rela- 

 tive geological position of these two rocks is not absolutely 

 constant. M. Pareto assures us that in Liguria the alberese 

 is always superposed on the macigno ; I have apparently seen 

 the same thing in Tuscany, but in some localities the two rocks 

 alternate and are mingled together. The fossils which most 

 generally characterize the macigno are Fucoids, the most 

 abundant species of which are F. intricatus, furcatus, Targioni. 

 With regard to animal species, they are of extreme rarity : 

 Nummulites have been found at Mosciano, near Florence, and 

 I have met with some at Alberona, in the Pouille. 1 must 

 mention another very important fossil found by the celebrated 

 Micheli in the pietra forte of Florence ; it is a fragment of a 

 chambered shell, which, by the contour of the spiral, seems to 

 belong to a Hamites, or perhaps to an Ancyloceras. This 

 valuable fossil has been preserved in the collection of Tar- 

 gioni at Florence, where it was observed by Brocchi*, by 

 Nesti, and by Savi ; the last also took a cast of it in plaster, 

 which is now in the museum of the University of Pisa. It 

 is truly to be regretted that the specimen I speak of has been 

 lost in the changes which the above-mentioned collection has 

 undergone. At the congress held at Milan, my friends MM. 

 Pentland and Pareto assured me that they had found two 

 * Conchiologia Fossile Subapennina, tomo i. p. 17. 



