i68 The American Geologist. ^f'^'''"' i»o^ 
The roof of the Perry cave is very interesting in that 
it is gently arched. Many of the strata, composing it, do 
not, however, extend entirely across the cave. The roof, 
therefore, in some places has an appearance similar to in- 
verted steps. The floor conforms to the uneveness of the 
roof, for where there is a depression in the former there is a 
corresponding projection downward in the latter and vice 
versa. This phenomenon is, doubtless, the result of fold- 
ing and subsequent collapse. 
The question, which must be settled first, is what has 
brought about the rock disturbances inasmuch as there is 
no evidence of volcanic action. During the past summer 
several wells were drilled on the island. The one on the 
farm of Louis Schiele on the southeastern shore of the 
island reached a depth of 170 feet. Below a depth of 100 
feet alternate layers of gypsum, shale, and limestone were 
encountered. Some of these deposits of gypsum were 10 
feet thick. The cores* between the depths indicated show 
that a very large amount of brecciation has taken place, giv- 
ing evidence to the fact that the gypsum is the result of 
the hydration of anhydrite. 
The hydration of anhydrite is always accompanied by 
an increase in volume. Credner.t Fritsch,* Bauer,§ and 
Geikie II estimate the increase in volume, thus brought 
about, to be approximately 33 per cent. On the other hand, 
however, such reliable authorities as J. D. Dana** and Nau- 
mann-Zirkel "^t place this increase in volume at a much 
higher figure, namely 60 per cent. J- Roth,M moreover, 
has carefully calculated the change in volume, which must 
result when anhydrite takes up two molecules of water of 
crystallization to form gypsum, and estimates the same to 
be as high as 62.3 per cent. Perhaps, in some instances, 
values between those just indicated would represent the 
true amount of increase. 
* The well was sunk by means of a socalled "shot drill" about three 
Inches in diameter. 
t Geologic, 8te Auflage, 1897, 221. 
tDas Gotthardgebeit. Bern, 1873. Anhjdrit und Gyps, 119. 
5 Lehrbiich der Mineralogie, 2te Auflage, 1904, 847. 
U Textbook of Geology, 4th Edition, 1903 T., 453. 
*•■ Manual of Geology, 4th Edition, 1895, 138. 
It Elemente des Mineralogie, 13te Auflage, 1898. 546. 
tX Chemische Geologie, 1879, I, 89. 
