Abstract
The two drill‐holes that the Geophysical Laboratory put down in the geyser‐basins in Yellowstone Park in 1929 and 1930 gave Information on a number of matters. In the present article only the mineral changes that the rocks have undergone, and their significance, will be discussed. The results of this study fit in most satisfactorily with those that E. T. Allen obtained from his studies of the mineral content of the discharge‐waters of the thermal springs.

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