208 T. 8. Huni—The Decay of Rocks. ‘ 
Mineral Resources of Wisconsin.” In Jackson and Wood 
counties, where the crystalline (Laurentian) rocks are covered 
a thin sheet of Potsdam sandstone, the river-valleys, 
cutting through this, expose the kaolin, which “occupies its 
original position, retaining sometimes the structure of the un- 
altered rock.” ‘This is derived from the decay 7m sztwu of cer- 
tain bands, which passing downward, graduate into unaltered 
feldspathic rock. Save where this mantle of decayed material 
has been protected by the Paleozoic sandstone, the crystalline 
rocks are there seen for the most part in an undecayed.condi- 
the EHozoic crystalline rocks was already far advanced in pre- 
Cambrian times m informed that similar evidence 1s 
afforded in Sweden by the presence of decomposed rock be- 
that the sculpturing of the gneiss rocks of western Scotland, a 
process which I have maintained to be dependent on previous 
sub-aérial decay, was.effected before the deposition of the Cam- 
brian sandstones, which there rest upon ancient roches mou- 
tonnées.” 
. Dawson. | 
Another instance is afforded by a dyke intersecting the 
5? Proc. Amer. Inst. M. Engineers, viii, 103. 
58 Nature, August 26, 1880, p. 403. 
