352 
Quaternary Deposits , Etc. — Mills . 
softened gneissoid rock without striking sound rock. I sup¬ 
pose the upper 4 metres in the one case, and the upper 20 
metres in the other to be in Quaternary loose materials. 
From this it is plain that the softening in the Rio Grande 
do Sul has been produced on a very large scale since the Car¬ 
boniferous strata were deposited and raised above the sea lev¬ 
el. The region of the coal basin of the Arroio dos Ratos is 
comparatively low and level, with gentle slopes, though as be¬ 
fore said this part of Rio Grande do Sul has probably been 
somewhat elevated in Quaternary or recent time. 
In Minas Geraes the softened layer occurs over a region of 
much greater elevation, of steep slopes and of V shaped drain¬ 
age channels, and this too while it is a region of abundant 
rainfall. Frequently and over considerable area the steep- 
sloped surface of the softened rock bears a meagre vegetation 
although in a tropical climate of abundant moisture. The 
vegetation is of feeble growth there because the soil proper, 
that is, the soil enriched by organic matter, is washed off. 
In a country of abundant rainfall the V shape of the drain¬ 
age channels is proof of comparatively recent elevation, but it 
seems impossible that the softened layer could have been 
formed or could long have existed at the present elevations 
with the present steep slopes and rapid erosion. The water 
runs off quickly and under a tropical sun the ‘evaporation is 
great; the drainage from the upper portions of the rocks also 
must be comparatively free where deep ravines abound. The 
conditions are therefore not favorable for deep penetration 
and saturation of the rocks by water carrying the carbonic 
acid necessary for rapid decomposition, while the conditions 
are very favorable for rapid erosion, and the softened rock is 
very easily eroded, and in fact has been eroded away leaving 
the solid rock bare in many places. For these reasons I con¬ 
clude that the region was raised above sea-level and worn down 
to a “base level of erosion 55 at an early period, and remained a 
low-lying region for a very long time, and has been within 
Quaternary or recent time raised to its present elevation. This 
conclusion is confirmed by other reasons to be hereafter 
given. 
On the surface of the rocks,—softened where consisting of 
gneisses and slates, and hard where consisting of sandstones 
and other resisting materials,—rest the Quaternary deposits. 
