_Ground Ice._--Has any satisfactory explanation been given of the mode
in which the peculiar substance termed _ground ice_ is formed in certain
rivers. I am most familiar with it as seen in the Wiltshire Avon. It is
seen in some rivers in Lincolnshire, where I am told it is called
_ground-gru_. One who has noticed it in the Teviot says, that the
inhabitants there call it "sludge."
The fact of ice being formed at the _bottom_ of streams, where we should
expect a higher temperature, is so curious an anomaly, that it would be
desirable to collect instances where and at what depths it is observed.