Quaternary Deposits, Etc. — Mills. 351 
generally has a flaky, felted structure, and which is called in 
the country '"Jacutinga." 
The gneiss and slates are softened to great depths from the 
surface. I have seen sections showing a thickness of over 100 
feet (estimated with the eye) of this softened rock, and yet not 
reaching to the bottom of it. The softened mass retains to a 
great degree the lamination and other structure of the rock. 
The quartz veins imbedded in it remain comparatively in- 
tact. 
The softening may be accounted for in part by the abund- 
ance of animal life in the soil overlying these rocks. Ants es- 
pecially occupy the soil everywhere. They are continually 
pouring carbonic acid gas into the upper layer of loose mater- 
ial, which the abundant rain-water washes down into the 
rocks, and carbonic acid is the great decomposing agent in 
rocks, taking out lime, the alkalies, and iron oxides and start- 
ing a whole train of alterations. But even with the added 
agency of abundance of animal life these rocks must have 
been very long, geologically speaking, under the decomposing 
action of carbonic acid brought to them from the atmosphere 
by percolating waters. They must then have been for a long 
time not covered by the sea or other sheet of water but exposed 
to the atmosphere. As the softened rock is overlaid by the 
Quaternary cascalho and loess which have not been visibly 
softened since deposition, it is plain that the softening was 
mostly produced before the time of the deposition of the loess 
at least. The cascalho being mostly of resisting material 
would not show effects of decomposition. The rocks them- 
selves are of Archasan or of Archaean and early Paleozoic age, 
consequently the softening has certainly taken place between 
Archaean and Quaternary time. Prof. 0. A. Derby ^ gives an 
account of borings, through great thicknesses of this softened 
material in the coal basin of the Arroio dos Ratos in the prov- 
ince of Rio Grande do Sul. One boring passed through first, 
4 metres of clayey soil, then through 120 metres of softened 
strata, and then went 17 metres farther in material sufficiently 
hard to be called stone. A part if not all of the softened strata 
are certainly of Carboniferous age. Another boring went 
through 20 metres of clay and sands with a gravel bed, then 
through decomposed shales 60 metres, then 18 metres in 
1 American Journal of Science; third series, vol. 27, p. 130. 
