Rdatloii of Secular Decay of Hods:. — Tarr. 29 
which, though originally so hard that it will ring under the ham- 
mer has been transformed to a yellowish cla}' that can be moulded 
like putty. Fresh sections to a depth of forty feet show this de- 
cay to the bottom, but the enclosing walls of sandstone show no 
alteration. The resulting product is essentially a kaolin Avith 
about seventeen per cent, of ferric oxide.* 
Among the other instances of secular rock deca}' in the United 
States might be mentioned that of the Ozark mountains of Mis- 
souri, described by Prof . Pumpelly.t Here the secular dissolving 
away of the limestones, which carry from two to nine per cent, of 
silica and clay, has left residuary products varying in thickness 
from twenty to one hundred and twenty feet. 
The best development of secularly decayed soil is in the trop. 
ics. In Nicaragua such deca)' has often reached to a depth of 
two hundred feet, according to Thomas Belt, who says that this 
kind of decay is common in the tropics and is chiefly confined to 
regions of forest. + 
The remarkable decay of the rocks in Brazil, which, with the 
associated phenomena, led Louis Agassiz to ascribe its accumula- 
tion to glacial action has been described by various writers. Dar- 
win states^ that in the vicinity of Rio Janiero both the granitic 
(metamorphic?) rocks and talcose slates are decayed to a great 
depth. Every mineral except the quartz is softened sometimes to 
a depth of one hundred feet, but the foliation of the rock remains. 
Appalled. by the vastness of the phenomenon, seemingly unlike any- 
thing at present in operation, he ascribed its formation to action 
beneath the sea. Of the same region Hartt says|| that the gneiss 
hills are covered with a coat of red soil, structureless and unar- 
ranged, varying in depth from a few feet to one hundred feet. 
Sometimes there are included in the clay angular fragments of 
quartz and rounded masses of diorite, generally quite decomposed 
except at the ver}- core. The gneiss beneath is decomposed to a 
depth of from a few feet to one hundred feet, the feldspars hav- 
ing been altered to kaolin, the mica having parted with its iron, 
but the planes of stratification still remaining. 
*Insoluble alumina Alg O3 and Fe^ O3 increase, but Si Oj, Ca O, Na^ 
O, Fe O, etc., decrease by this decay. 
tAm. J. Sci , 1879, xvir, 133-44. 
JXaturalist in ISicaragua, 1888, p. 86. 
gGeological observations, p. 427. 
II Geology and Physical Geography of Brazil, 1870, pp. 23-26. 
