196 T. S. Hunt—The Decay of Rocks. 
sive elimination of the alkalies from the aluminiferous rocks of 
the eozoic groups was shown by comparing the mineralogical 
composition of the Laurentian with the Huronian, Montalban 
and Taconian crystalline schists.” 
$16. It should here be noted that decayed feldspars, ie 0 
when these are reduced to the condition of clays, have not, 
most cases, lost the whole of their alkalies. This is well ines 
trated in a series of analyses by Mr. E. T. Sweet, of the kaolin- 
ized granitic gneisses of Wisconsin, to be noticed farther on 
From these analyses it appears that the levigated clays 
from these decayed rocks still hold, in repeated examples, from 
two- to three- hundredths or more of alkalies, the potash pre- 
dominatin 
17. It would follow from the considerations advanced 
above, that the decay of crystalline rocks is an indispensable 
preliminary to the formation of limestone, and that the earliest 
silicated rock must have contained no carbonates whatever. 
There are many reasons for doubting whether this veritable 
primary system is known to us, but it will be remembered that 
at the base of the Laurentian, as seen on the Ottawa River, 
there is a vast and unknown thickness of red and vray granitoid 
hornblendic gneiss, apparently destitute of limestones, an 
underlying the great series of somewhat similar gneisses inter- 
stratified with limestones wale graphite, iron-oxyds an 
S Regs which were, as early as 1847, reas by the Cana- 
ian geological survey as “a separate group of metamorphic 
strata,” characterized by these lithological aitlcne aaa Logan, 
however, supposed the two groups to be conformable, and they 
were in 1854 both included under the common name of Lauren- 
tian (afterwards the Lower Laurentian of Logan 
in a subsequent review of the subject in 1878, I designated 
the lower as the | Ottawa group, and the upper group as the 
Grenville series.“ It was this latter group which had been 
carefully studied and mapped by Logan and his assistants, and 
as served as the type of the Laurentian system. I have 
since, in a paper read before the American Association for the 
Advancement of Science, in 1879,* noticed the probable-non- 
conformability of this Laurentian system with the underlying 
or pre-Laurentian gneiss, and the possible relations of the latter 
to the fundamental or Lewisian gneiss of Scotland, = the 
Bojian gneiss of Bavaria. 
2 Second phanie Survey of Pa.; Azoic Rocks, Rep. B, p. 2 
38 See for on the Min eral Resources of Wineobis Proc. Amer.. 
Inst. M. Helin vol. oe at 305. For ae analyses, see Geo. H. Cook, Geol. 
: Survey of New J only rt on Clays, 18 
24 Azoic oe 64, 148, pete 
_ %The pre-Cam ins Rocks 0 f Europe and America compared. Amer. Jour. 
Sci., xix, pp. aie, 275, 281; also Geol. Mag., Jan. 1882, p. 38, and Bull. Soc. 
Géol. de France, x, 26. 
