358 The Aniericafi Geologist. Juue, looo 
in the waters of the lake. The average amount of this gas 
contained in water is given by Fisher* as about 3.35 cc. 
per Utre. As the amount of carbonic acid in the atmosphere 
along the Atlantic coast is fairly representative of average 
conditions, the quantity contained in the waters of lake Win- 
nipesaukee woidd probably not vary greatly from this amovmt. 
Notwithstanding the low percentage present, it was to the 
solvent action of this carbonic acid that the differential 
weathering was at first referred, the unusual rapidity and ex- 
tent of the action being considered as the result of the pecul- 
iarities of the rock. 
The gneiss consists mainly of feldspar and biotite, the lat- 
ter being present in large amounts, and giving the rock an 
almost schistose character in places. This fissility assures 
the water an easy access to the rock for some distance below 
the surface, and probably is an important factor in the weather- 
ing. Both the feldspar and the biotite show evidences of 
previous alteration, the former by its whitening, the latter by 
its greenish color. It is a well known fact that alteration 
once started wall often proceed with great rapidity, even under 
unfavorable circumstances, and it is probably to this already 
existing alteration of the minerals that we owe the present 
somewhat rapid erosion of the gneiss. 
Since the foregoing observations were made the writer, 
through the kindness of Prof. W. O. Crosby ,f has had an op- 
portunity to examine a number of specimens of a disinte- 
grated biotitic granite from Colorado,* which seem to throw 
considerable light upon the nature of the process by which 
biotitic granites and gneisses may disintegrate in positions 
which preclude the influence of changes of temperature or of 
frost action as important factors in the action. The Strickler 
tunnel from which the specimens were taken, forms a part of 
the water system of Colorado Springs, and is situated at an 
altitude of some 11,500 feet. In it, at a depth of some 300 
or 400 feet below the surface^ are to be found large amounts 
of a coarse red biotitic granite in all stages of disintegration. 
On examining specimens in which the disintegration is just 
*Die Chemische Technologic der Wassers, Brauschweig, 1880, pp. 
75-76. 
tThe nature of the disintegration was recognized by Prof. Crosby 
several years ago, and was personally communicated to Prof. G. P. 
Alerrill. (See "Rocks, Rock Weathering, and Soils", 1897, p. 189). 
