DUROCIIER ON SOILS AND VEGETATION. 35 



distinctly narrower than the others, with which they perfectly agree 

 in all other respects. M. Barrande is inclined to refer this distinc- 

 tion to difference in sex, the narrower specimens being males, the 

 broader females. 



The same genus, Odontopleuray shows the care necessary in form- 

 ing new species ; almost every individual has a different number of 

 spines on the tail, so that this forms no ground for distinguishing 

 species. Even the nature of the stone in which they are imbedded 

 produces remarkable differences : thus in the slates all the projecting 

 ridges, &c. are pressed flat ; in the quartzites, on the contrary, they 

 preserve the same prominence as during the lifetime of the animal. 



The compoimd eyes are in many specimens singularly well pre- 

 served. In the eye of a Brontes palifer M. Barrande counted nearly 

 30,000 lenses. [J. N.] 



Observations on the Relatioti between the Mineral Character of the 

 different Formations and their Vegetable Productions. By 



M. J. DUROCHER. 



[Comptes Rendus, torn, xxvii. p. 506.] 



In the course of the many years that I have been engaged in ex- 

 amining the west of France, I have observed numerous facts regard- 

 ing the influence of the mineral nature of the different formations on 

 the development of plants, of which the following are a few : — 



In an agricultural point of view, the formations composing the 

 subsoil of Brittany and the neighbouring districts may be divided, 

 without regarding their geological age, into five classes : 1 . granite 

 and crystalline schists (of granitic elements) ; 2. clay slates and grey- 

 wackes ; 3. quartzite or sandstone, and quartzose schists ; 4. tertiary 

 deposits of an argillaceous-gravelly or pebbly nature; 5. calcareous 

 formations. In another respect three great agronomique divisions 

 may be formed : 1. cultivated ground and meadows ; 2. the forests ; 

 and 3. the landes. During my geological studies I have determined 

 the manner in which the landes and forests are distributed over the 

 surface of these various formations. I have remarked that in Brit- 

 tany and the surrounding countries they are generally confined to two 

 kinds of formations — to the argillaceous-pebbly tertiary deposits, and 

 more especially to the quartzite and quartzose schists. The latter 

 variety of soil, though comparatively it does not occupy a very large 

 extent of surface, still in many departments presents a greater extent 

 of landes and forests than all the other formations conjoined. Landes 

 and forests are also occasionally observed on granite, principally in 

 Morbihan ; but they are less frequently seen on the clayslate and 

 greywacke, and very rarely on the calcareous formations. 



The peninsula of Brittany presents four zones well-characterized by 

 their geognostic and agricultural characters : a littoral zone, com- 

 prising the two coasts north and south, formed chiefly of granite and 

 crystalline schists ; secondly, a central zone composed of clay slates and 

 greywackes, sprinkled with a few tertiary deposits ; and lastly, the 

 two zones that separate this central band from the coasts, consisting 

 of quartzose rocks, intermixed with schists and some granite masses. 



