TIME OF WEATHERING. 267 
But in the Adirondacks, just to the north of this locality and in essen- 
tially the same climate, there are abundant acid rocks, in dikes and 
bosses, in a very fresh condition, although they have been exposed to 
weathering agents quite as long as the dike in question. The syenite 
dikes described by Professor H. P. Cushing in the present volume afford 
a concrete example, showing as they do only very slight evidence of 
weathering. ; 
The conclusion is obvious that to reduce one of these acid rocks to the 
condition of the weathered alnoite will require a vastly longer period 
than has been needed in the case of the latter rock, and during all of that 
time the biotite of the acid rock will be undergoing decomposition. It 
follows that the decomposition of the biotite will go on almost simul- 
taneously with the slow weathering of the acid rock as a whole, while it 
may lag far behind the much more rapid weathering of the basic rock, 
- even though the actual rate of change in the biotite be the same in both 
cases. : 
That this is a true cause, though probably not the only cause, of ap- 
parent differences in the rate of weathering of biotite in acid and in basic 
rocks seems, at least, a justifiable hypothesis. 
Time or WEATHERING 
The position of the dike is such as to fix the period during which the 
weathering has been accomplished within narrow limits. As above 
stated, the dike is cut through by the narrow rock-walled gorge of the 
Hast Canada creek, which is assumed to be of unquestioned postglacial 
age. The weathered portion of the dike is on the convex side of a bend 
in the gorge precisely at the point that must have received the full force 
of the current in the early stages of the gorge-cutting. That the dike in 
its present condition could not resist this current for an hour, but would 
be scoured out as far as the weathering penetrates, is perfectly obvious, 
and the presence of the weathered material in'situ is positive proof that 
it was not in its present condition when the upper part of the gorge was 
cut. On the contrary, it must have been nearly as tough and resistant 
as the surrounding “sand rock,” or, in other words, in the unweathered 
condition shown on the east bank of the stream. 
The very considerable degree of weathering here exhibited has been, 
then, accomplished in postglacial time; and this, too, in a temperate 
climate. Much stress has been laid upon the impetus given to weather- 
ing by tropical heat ; but here is another instance to be added to those 
