DECOMPOSiriON OF ROCKS IN BRAZIL 533 



facilitated h\ the fault [)lanc and by any secondary shattering 

 that may attend it. It may thus happen that in the distance of 

 a few feet the same rock may be found almost perfectly sound 

 antl profoundly decomposed, and in cases of apparently abnor- 

 mal tlccom[)osition the local conditions should be carefull}^ 

 studied before special causes (which will often be found to have 

 respected adjacent rock masses) are called into play/ 



Of the greater part of the examples that have been cited as 

 showing the extraordinary extent of rock decay in Brazil no 

 details regarding petrographic character and topographical posi- 

 tion hax'e, or can be, given from which an opi'nion can be formed 

 as to whether or not the decay is normal or abnormal in char- 

 acter. The two exceptions are the cases of the Pedregulho 

 reservoir in Rio de Janeiro and of the new shafts of the Morro 

 Velho mine in Minas Geraes for both of which the figures are 

 definite and the conditions are such that the decay observed 

 may be considered as about normal for the kinds of rocks con- 

 cerned. 



' The subject of faulting has received very little attention in the study of Brazilian 

 geologv, but it is becoming apparent that many of the topographical features are due 

 to faults and their determinative effects upon erosion rather than to erosion pure and 

 simple as generallv assumed. The evidences of faulting are most apparent in the 

 regions of horizontal sedimentaries and bedded eruptives of the southern part 

 of the country, but there are reasons for believing that they will also be found, 

 perhaps on an equal scale, when detailed studies are made of the more ancient 

 regions of eastern and central Brazil and of the more modern ones of the north- 

 ern part of the country. For the latter region the possibility of faulting was sug- 

 gested, rather too obscurely, as it appears, in a brief resume of what was known of the 

 Cretaceous in Brazil prepared by the present writer to accompany the monograph of 

 Dr. C. A. White on the fossils of that age (Archivos do Museu Nacional do Rio de 

 Janeiro, Vol. VII). The paragraph in which attention was called to the comparatively 

 low level of the fossiliferous coastal basins as compared with the beds of the interior 

 referred to the same age but with, so far as kno>\'n, a different fauna, suggestive of 

 a possible rise of the land occurring between the two faunas, has been strangely 

 misapprehended bv Dr. Branner who characterizes it as a hypsometric classification of 

 formations. (The Cretaceous and Tertiary Geology of the Sergipe-Alagoas basin of 

 Brazil, Trans. Am. Phil. Soc, Vol. XVI, 1889, p. 410.) As he unites the two faunas 

 on no better ground than the attempted refutal of the supposed low level of the coastal 

 basins, the epithet may perhaps with greater justice be applied to his own reasoning. 

 The refutal in question is contained in the statement that the Cretaceous beds extend 

 to within 1 50 meters of the top of the Serra de Itabaiana which, according to Mouchez, 



