448 EDSON S. BASTIN 
since a large proportion of Sodium in the original igneous rocks occurs in minerals 
which are more readily decomposed than the minerals which bear potassium— 
that is, sodium occurs largely in the nephelites, sodalites, and basic feldspars, 
which are readily soluble; whereas the great sources of potassium are ortho- 
clase and microcline, difficultly decomposable minerals. The materials are also 
apt to be depleted in calcium and magnesium, since the alkaline earths are so 
readily soluble. The depletion in calcium usually has gone farther than the 
depletion in magnesium, since in the belt of weathering much of the magnesia is 
retained in the serpentines and talcs. The material may or may not be depleted 
in iron. While aluminum and silica also have been dissolved in the belt of 
weathering, the solution of these substances is less rapid than of the others, and 
thus there is usually an increase in the relative amounts of these elements. 
In discussing the analysis of a slate from the Menominee district, 
Mr. J. Morgan Clements says: 
That which is most striking about the analysis is the relative proportion of the 
alkaline earths, lime and magnesia, the latter being present in the greater quantity. 
As a rule, in all of the igneous rocks (and to the igneous rocks all clay slates 
owe their ultimate origin), except in the non-feldspathic ultrabasic ones, the 
reverse condition exists, namely, the magnesia is subordinate in quantity to the 
lime. 
Dr. F. D. Adams? remarks as follows: 
When any granite or granitic rock is for long periods exposed to a process of 
gradual decay there finally results a mass of kaolin, often mixed with more or 
less chloritic material, holding the quartz of the original rocks as grains scattered 
thickly through it. When sorted by the action of moving water it gives rise to 
beds of sand and clay. The chemical processes at work during this process of 
decay consist in the more or less complete removal of the alkalies of the feldspar, 
and of the decomposition of the iron-magnesia constituents with the loss of a 
large proportion of the lime as compared with the magnesia. 
On comparing the analyses of a series of granites and those of a series of slates, 
as for instance those given in Roth’s Gesteins Analysen, the latter are seen to be 
on an average considerably higher in aluminum and much lower in alkalies, 
while at the same time they are lower in silica which has been separated both 
as sand and in combination with the alkalies which have gone into solution, and 
in most cases contain more magnesia than lime instead of more lime than magnesia 
as is usual in the granites. 
The characteristics of foliated rocks of sedimentary origin which 
have been considered useful in distinguishing them from foliated 
rocks of igneous origin may be summarized as follows: 
t Monograph XXXVI, U.S. Geol. Survey, pp. 59, 60. 
2 F. D. Adams, ‘‘Contributions to Our Knowledge of the Laurentian,’’ Amer. 
Jour. Sci., 3d series, Vol. L, pp. 64, 65. 
