Ancient Beds of Torrents. 117 



to that whose traces are visible on all the great mountains of 

 Europe. These facts are remarkable in themselves, and the 

 approximations to which they may give rise seem to us wor- 

 thy of all the attention which the author has bestowed on them. 

 They will remain as landmarks, without doubt still too few 

 in number, in the midst of the discussions which they will not 

 fail to originate. 



General Observations. — From all that we have said, it thus 

 appears that the stratified formations of South America may 

 be divided, according to M. D'Orbigny, into eight very dis- 

 tinct groups : — 1. The old crystalline formations, in w^hich 

 gneiss predominates ; 2. The Silurian and Devonian transi- 

 tion formations ; 3. The Carboniferous formations ; 4. The 

 Triassic formation ; 5. The Cretaceous formations ; 6. The 

 Guaranian and Patagonian tertiary formations ; 7. The Pam- 

 pean loam ; 8. The modern deposits, which he also terms di- 

 luvial, from the nature of the cause which has produced them. 



These different groups of beds have positions which are al- 

 together dissimilar and discordant ; and, according to M. D'Or- 

 bigny, these discordances result directly from the dislocations 

 which have changed the surface of America, and have given 

 rise to the chains of mountains which traverse it. In the same 

 manner as has been attempted in Europe, and as M. Pissis 

 has attempted in regard to Brazil,* M. D'Orbigny has endea- 

 voured to connect the interruptions of continuity presented 

 by the series of American formations, with the successive ap- 

 pearance of the chains of mountains which form the principal 

 features of the relief of South America. His classification 

 embraces two systems of mountains already pointed out by M. 

 Pissis. As we have already said at the commencement of 

 this report, a very old gneiss formation presents itself over a 

 great extent of country on the eastern coast of South Ame- 

 rica. It occupies the eastern portion of Brazil, to the east of 

 the Mantiquiera, from the 16th to the 27th degree of S. lati- 

 tude, and there forms a series of small chains, whose general 

 direction is, according to M. Pissis, from east 38° north, to 



* Vide the Rapport on the Memoir of M. Pissis, Comptes rendus, 

 t. xvii. p. 28. 



