SEDIMENTARY FORMATIONS. 
33 
of the rock during alteration; or (2) that calcite has been taken out 
of the fresh limestone, leaving the silica and alumina in larger pro- 
portion, in which case the weight and, as will be shown later, the 
volume has considerably decreased; or (3) some combination of these 
two methods has accomplished the same result. Alternative 2 is fa- 
vored by the fact that, while the alumina has gained slightly in per- 
centage on the silica, the ratio of the two substances in the fresh and 
altered rocks remains substantially the same, in the fresh rocks average 
alumina standing to silica as 19 to 100, and in the altered rocks as 
21 to 100. (See analyses, pp. 26-27, and PI. X.) The near accordance 
of the alumina and silica circles in PI. X brings out well the uni- 
formity of the ratio. If silica and alumina have been added from 
without, it is a remarkable coincidence that they should have been 
added in so nearly the same proportion as in the fresh rock, especially 
when it is remembered that their proportion in the fresh limestone 
is not determined by silicate ratios but rather by the abundance of 
chert. 
On the assumption that alumina remains constant, it will be noted 
that the greatest percentage lost is calcium, next magnesia, then 
potassa, and then silica, which is the order of solubility of these sub- 
stances in weathering. The only exception is soda, which without 
much question has been introduced into the contact phase. It will 
be noted further that the composition of the altered rock is essen- 
tially that of a calcareous residual clay. 
Comparison of average fresh Homestake limestone with normal contact phases, with 
alumina (Al 2 3 ) assumed to be constant. 
A, B. 
H. 
I. 
J. 
H, 
r, j. 
Varia- 
Varia- 
Varia- 
Varia- 
Average, 
fresh. 
Altered. 
tion 
from 
Altered. 
tion 
from 
Altered. 
tion 
from 
Average. 
tion 
from 
A,B. 
A, B. 
A, B. 
A, B. 
Si0 2 
7.49 
5.168 
- 2.322 
8.313 
+ C.823 
8.62 
+ 1.13 
7.367 
- 0. 123 
A1 2 3 
1.49 
1.49 
0.C0 
1.49 
0.00 
1.49 
0.00 
1.49 
0.00 
Fe 2 3 
.95 
1.172 
+ .222 
.812 
- .138 
.468 
- .482 
.817 
- . 133 
FeO 
.41 
.115 
- .285 
.385 
- .025 
.271 
- .139 
.290 
- .120 
MgO 
3.69 
.648 
- 3.042 
1.502 
- 2.088 
1.233 
- 2.467 
1.127 
- 2.563 
CaO 
46. 91 
.126 
-46.784 
2.313 
-44.597 
1.301 
-45.6C9 
1.243 
-45.665 
Na 2 
• .13 
.206 
+ .076 
.310 
+ .180 
.649 
+ .519 
.388 
+ .258 
K 2 
.85 
.432 
- .418 
.225 
- .625 
.236 
- .614 
.298 
- . 414 
H 2 + 
1.03 
.716 
- .314 
.527 
- .503 
.588 
- .442 
.610 
- .419 
P 2 5 
.04 
.032 
- .008 
.019 
- .101 
.019 
- .021 
.023 
- .043 
C0 2 
37.01 
.021 
-36.989 
.100 
-36.910 
.263 
-36. 747 
.128 
-36.882 
100.00 
10.126 
89. 864 
15. 996 
83.984 
15. 138 
84 872 
86. 104 
-13.791 
Exceptional contact phases. — These phases represent introduction 
of calcite, iron, and silica. Analysis L is of an iron-stained marble 
in a vein. Probably all of the constituents are secondary, but if the 
alumina or silica is the same as in the fresh limestone, there has been 
a considerable introduction of iron and a slight loss of calcite and 
soda, or if alumina has remained constant there has been also a gain 
28463— Bull. 338—08 3 
