I)/-:COMPOS/770.V OF ROCKS /\ P.RA/.ll. 539 



An opportunity for observing the phenomena of decomposi- 

 tion iHulcr substantially the same climatic and biologic condi- 

 tions but with reference to a totally different series of rocks is 

 afforded by the recent notable extension of coffee culture in the 

 interior of the state of Sao Paulo with the accompanying exten- 

 sion of the railway system. No critical study of this region has 

 as yet been made and, as in nearly all that precedes, the obser- 

 vations (whether those of the writer or of others) must be taken 

 as railroad geology and, as such, subject to future revision. 



The region in question lies immediately to the west of that 

 above considered and in so nearly the same conditions as regards 

 latitude, elevation, rainfall, and forest distribution that the dif- 

 ferences may be considered as unimportant for the present dis- 

 cussion. The features of geological and topographical structure 

 and of the character of the rocks are however widely different. 

 The dominant geological and topographical features are given 

 by soft shales and sandstones of late palaeozoic (Permian?) and 

 early secondary (Triassic?) age which are practically horizontal 

 though much disturbed by faults and dykes and sills of basic 

 eruptives. The latter belong exclusively to the group of augite- 

 porphyrites as defined by Rosenbusch and represent all the 

 varied phases of that group. According to all the indications at 

 present known the region has stood as dry land ever since early 

 secondary times. Much of it is in campo but considerable forest 

 tracts occur. No direct relation can be traced between the dis- 

 tribution of forest and campo and the character of the under- 

 lying rocks, though in general it may be said that campo pre- 

 dominates over forest in the areas in which the sedimentaries 

 give character to the surface soil, the contrary being the case 

 where the eruptives come to the surface. 



In this region the evidences of profound decomposition are 

 far less prominent than in the mountainous districts above 

 referred to. The railroad cuttings are not deep, seldom exceed- 

 ing a half dozen meters, but they rarely fail to show some rock 

 in a comparatively sound condition which frequently extends to 

 within a meter or less of the surface. The sedimentary rocks 



