Siib-Aqiieous Differential Weathering. — Fuller. 357 
certain narrow limits by the dam at Lakeport, can never have 
been much lower than at the time the observations were made, 
which was at the close of an unsiially dry summer when the 
water was much below its ordinary level. 
The considerable depths at which many of the ledges occur 
rules out the alternation of heat and cold as an important 
cause of disintegration, for the changes in the temperature 
of water at a depth of ten feet would be very gradual and 
mostly seasonal in nature. Freezing of the water contained 
in the pores of the rock is also ruled out, except in those 
ledges lying near the surface of the water. As differential 
weathering is no more marked in such instances than in the 
ledges of greater depths, it is evident that freezing has not been 
an important factor in the production of the observed results. 
At a depth of ten feet, the waves of the lake — never verv 
high — would be made manifest only by a slight oscillation 
of the water particles, sufficient, perhaps, to move fine sedi- 
ments, such as the products of disintegration, but incapable 
of performing any work of erosion. 
It is. then, to the water itself, that we must turn in seek- 
ing" an explanation of the differential weathering. Even here, 
however, the cause is not at first apparent, for the waters of 
lake Winnipesaukee are of unusual purity, and appear to con- 
tain nothing to which the erosion can be referred. The lake is 
fed mainly by short streams from the surrounding hills and 
mountains. Testifying to its purity is the fact that while 
ordinary rain water contains, on an average, four parts in 
100,000 of solid impurity, the waters of lake Winnipesaukee 
carry only 2.12 parts. Free ammonia is present only in the 
proportion of i part in 500,000,000, chlorine i part in 83,000, 
and nitrogen (as nitrates) i part in 27,500,000.* The water is 
practically colorless, always very soft, and contains little or- 
ganic matter. 
The absence of sulphuric acid, and the small amounts of 
chlorine and nitrogen, eliminates these substances from the 
list of possible agents of chemical erosion. The only agents 
remaining are pure water and carbonic acid. Unfortunately 
no analyses could be found giving the percentage of the latter 
♦Calculated from the mean of twenty-four analyses. Massachusetts 
State Board of Health, Report of Water Supplies and Inland Waters, 
i8qo, pp. 447-448. 
