266 C. H. SMYTH, JR.—WEATHERING OF ALNOITE. 
The chemical changes thus far effected agree closely in most respects 
with the phenomena generally observed in such cases, the only unusual 
features being the easy solubility of the weathered rock and the oblitera- 
tion of the commonly stable alteration product, serpentine. 
CONTRAST BETWEEN WEATHERING AND ALTERATION 
In certain respects the contrast between weathering and alteration 
here presented is marked. The latter process led to the formation of 
much serpentine and calcite unaccompanied by any oxidation. Weath- 
ering has undone this work by removing both of these minerals, while - 
at the same time oxidation has been extensive. 
The only feature common to the two processes is hydration, whose 
effects are shown in both phases of the rock, although as a matter of 
fact the product of hydration by alteration has been destroyed during 
weathering ; so even here, as well, the two operations are in a sense 
antagonistic. The alteration can hardly be regarded as a destructive 
process, while the weathering is eminently such. 
Rate oF DEcomMposiTion oF BIOTITE 
The general rule that biotite, though in acid rocks one of the first min- 
erals to show signs of weathering, in basic rocks appears to be very resist- 
ant, finds excellent exemplification in this instance, as is evident from 
the foregoing. In the course of the investigation the question as to the 
proper interpretation of this rule has suggested itself. In the absence of 
any positive data as to differences in the weathering process or in the 
composition of the biotite in acid and in basic rocks sufficient to afford 
an explanation of the different rates of decomposition, the question has 
risen, Is this difference absolute or only relative? Or, in other words, 
does the biotite of a granite actwally weather more rapidly than that of a 
basalt, or does it simply seem to weather more rapidly because the other 
constituents of the granite weather more slowly than do those of the basic 
rock? Without presuming to answer this question upon the meager 
and uncertain evidence of a single instance, the writer would simply 
point out the bearing uponit of the casein hand. Here is arock weath- 
ered to a marked degree and yet containing abundant biotite, which, 
though much decomposed, is perfectly recognizable. It might be con- 
cluded at once that this comparative freshness of the biotite indicates a 
slow decomposition of the mineral due in some way to the fact that the 
rock is basic. 
