192 T. 8. Hunt—The Decay of Rocks. 
abrasion.” Like Fournet, of rejects the notion of an original 
concentric structure in the ro 
§ 6. Hartt, in 1870, pede the well-known examples of 
rock-decay found in Brazil, and called such rounded masses of 
rock as we have just described “ bowlders of decomposition.” 
He moreover noted that the process of decay was there Seales 
to the supposed glacial action, which had worked over the 
_ material of the previously decomposed rocks.’ | Lyell alee dps 
in 1849, had pointed out that the Tertiary clays and sands of 
the southern United States have been derived from the waste 
of the previously decayed crystalline rocks of the region ;* and 
as we have seen, the ante- ‘Tertiary age of the decay in Auvergne 
had long before been recognized. 
he account given by Professor Charles Upham Shepard, 
in 1837, of the origin and mode of occurrence of the porcelain- 
clays of western Connecticut is remarkable for its exactness 
and perspicuity. That at New Milford is described as occur- 
our author says, “« “11 forms a vein many feet in width, cut- 
vein of clay is described as occurring in the town of Cornwall, 
and as including frequent crystals of black tourmaline; the 
feldspar also being incompletely decomposed.° 
As showing that the process of arene decay is not con 
fined to silicated rock s, it may be noted that J. D. Whitney 
described in 1862, the existence in the lead-region of Wisconsin 
of a layer of red clay and sand, mixed with chert, sometimes 
thirty feet in thickness, which ‘he showed to be a ae 
ne ‘souri, where sak fis pete deo sometimes attain a thick- 
§ Geol. Recon. of California, pp. 146, 2 
: Facog Results of a Journey in sbeaail pp. 28 “573. 
Lyell: A Second Visit to the United Sta 28, 
: Boncerd: Report of ee Survey of Counsel (1837), pp. 73-75. 
18 Geology of Wisconsin, i, 121 
