1850.] ZIGNO ON THE VENETIAN ALPS. 425 



Mae, the Boite, to the upper part of the Cadore, whence it turns to 

 the east and follows the left bank of the Tagliamente. It is true 

 that the tops of the mountains beyond this line are sometimes of 

 Jurassic age, but the bottoms of the valleys always display the more 

 ancient rocks, which extend from hence, forming the principal part 

 of the mountains bordering the valleys of the Fiemme and the Fassa ; 

 so that in these two valleys, as well as in the transverse ones of Tra- 

 vignolo, S. Pellegrino, Livinal-lungo, and the well-known locality S. 

 Cassiano, the trias appears everywhere well-developed and marked 

 by its most characteristic fossils. The line I have just traced, which 

 is established by very careful observations of my own, as well as by 

 those which are scattered through the writings of MM. Pasini, Fuchs, 

 and CatuUo, obliges us to throw into the division of older rocks much 

 which is classed as Jurassic in the Maps of MM. von Dechen and 

 Morlot. The deep valleys in the neighbourhood of Belluno give op- 

 portunities of studying these rocks and observing their perfect con- 

 formability. In fact the Jurassic masses, which form the crests of 

 the high mountains of the Tyrol, the Bellunais, Cadore, and Friuli, 

 descend to the bottom of the valleys of the Piave and the Brenta, 

 where they are covered by the cretaceous and tertiary hills which 

 gradually subside into the plains north of Venice. They can be 

 studied in the valleys of the Brenta from Bassano to Borgo di Valsu- 

 gana ; of the Piave from Belluno to Perarolo ; the valley of Pantena 

 in the Veronals ; the road from Vallarsa to Roveredo, and thence to 

 Trent ; and the valley of the Astico in the Sette Comuni, as well as 

 in the transverse valleys which intersect this great calcareous plateau. 

 But before speaking of the Jurassic rocks, properly so called, I must 

 call attention to beds which are apparently older than the oolitic and 

 younger than the trias. Above the beds of the latter, brown or greenish 

 clay-slates, associated with a bluish grey limestone with spathose 

 veins, are found in several parts of our Alps, apparently older than 

 the oolites and belonging possibly to the lias. They occur in Cadore, 

 in the Bellunais, and in the Tyrol, and as they contain no fossils, their 

 place in the chronological series must be doubtful. They are not found 

 in the great valleys of the Piave, the Brenta, the Astico, the Agno, and 

 the Adige ; and in their place we find, beneath the undoubted oolite, 

 alternating beds of compact and crystalline limestone, apparently due 

 to some alteration of the strata which were deposited between the Keu- 

 per and the lower oolite or systeme Bathonien of M. d'Orbigny. The 

 crystalline structure proves that these beds have undergone plutonic 

 changes ; nevertheless wherever these beds are seen reposing upon 

 the trias, the latter shows no trace of metamorphism ; and I may 

 state generally, that all the sedimentary beds in these mountains, from 

 the mica slate upwards to these crystalline limestones, prove in the 

 clearest manner that they have undergone no alteration whatever from 

 plutonic agency, always excepting those partial changes in the imme- 

 diate vicinity of igneous rocks which are confined to particular locali- 

 ties and do not influence the general aspect of the formation. I can 

 only explain this metamorphism and the absence of fossils by attri- 

 buting it to an elevation of temperature and an evolution of gaseous 



