Sub- Aqueous Differential Weathering. — Fuller. 359 
beginning, a number of tiny cracks are observed beginning at 
the crystals of biotite and radiating outward irregularly be- 
tween the crystals of feldspar. On closer examination it 
becomes evident that the cracks are the result of the swelling 
of the crystals of biotite in consequence of the process of 
hydration to which they have been, and are now being sub- 
jected. The distance to which these cracks penetrate and 
their width evidently depend directly on the amount of hydra- 
tion the biotite has undergone. In the more advanced stages 
of hydration, which are marked by a more or less complete 
oxidation of the iron, the cracks are usually open, sometimes 
to a width of one or two millimeters, rendering the rock so 
friable that it crumbles in the hand. 
Although no opportunity has been presented for substan- 
tiating the view advanced below, it seems probable that the 
explanation of the disintegration of the biotite granite of the 
Strickler tunnel (i. e., hydration) will apply with equal force 
to the biotitic gneiss underlying lake Winnipesaukee. The 
observed chloritization of the biotite, as made evident by the 
greenish color and general aspect, is confirmatory to this view. 
The small strings and dikes of pegmatyte cutting the gneiss 
carry almost no biotite and very little muscovite. More- 
over, being of a later date, they escaped the powerful pressure 
to which the gneiss was subjected, and to which its lamination 
is probably due. For these reasons, they are more imper- 
vious to percolating waters than the surrounding rock, and 
their more acid character is less favorable to alteration. Such 
dykes would naturally show much less evidence of alteration 
than the surrounding gneiss, and would by their superior 
power of resistance be left in relief, as the latter was weath- 
ered away and removed. 
The remarkable feature of the occurrence described is 
that in the comparatively short period since the disappearance 
of the ice sheet from the region the process of hydration, or 
at least of sub-aqueous agencies, should have produced results 
quantitatively ec|uivalent to the combined results of the work 
of the atmosphere, rain, and frost upon similar ledges upon 
the land. The superior distinctness and perfection of the sub- 
aqueous chemical erosion is a natural result of the slow, but 
regular and persistent, and consequently more selective action 
of the water. 
