ON ICE PHENOMENA. 89 
temperature. In order to test this point, Mr. G. B. Abrey, D. T.$. an accurate scientific 
observer, a few years ago, undertook, at my request, to make a series of exact measure- 
ments of the width of a great ice-crack near his residence at Little Current, on Manitoulin 
Island. The result was that no change of width corresponding to changes in the tem- 
perature of the air was perceptible. It was hardly to be expected that a comparatively 
thin sheet of ice, resting on water of a uniform temperature, would be affected by changes 
in the temperature of the air, especially as it is protected from it by a coating of snow. 
Moreover, if it were a fact that the ice of rivers and lakes expands and contracts to any 
notable extent with changes in the temperature of the air, we should find a perceptible 
motion at the shore, wherever the ice-sheet is not relieved by fissures. But no such 
movement has been detected, and, as we have seen, no variation, due to temperature, 
takes place in the width of the fissures. What then is the cause of these fissures ? This 
question has not yet been satisfactorily answered, so far as the writer is aware ; but it may 
be suggested that the fissures are due to a falling of the water. They form every winter 
in the same situations and generally between the extremities of points on opposite sides of 
the water. A lowering of the wider body of water on each side of the line of the crack, 
would cause the ice to fall away from this line, and when the tension became sufficiently 
great, the fissure would form and would continue to widen with the progressive lowering 
of the water. It is well known that the waters of our large rivers and lakes begin to fall, 
after the frosts of winter have sealed up all the small tributaries, and that the process 
goes on until these feeders begin to run again in the spring. 
Another point in reference to river-ice may be briefly touched upon. It has been 
stated that the ice, which remains on some of our rivers for half the year, might exhibit a 
slight glacier-like tendency to move downward, especially in the centre of the stream. 
Opposite to the city of Montreal, where the current of the St. Lawrence is swift and the 
ice forms to a thickness of two feet, the writer tested this point during different winters, 
by making repeated observations on a series of marks set in straight lines across the 
river, but no deviation could be detected. 
Rines or BOULDERS.—Around the ponds in the rolling country of the second and 
third prairie steppes, these constitute a singular feature in the treeless regions of our 
Northwest Territories. Most of these ponds have uniform basin-shaped bottoms and 
circular or elliptical outlines. The boulders have been removed from their beds and 
deposited as rings all around their margins. These phenomena, which must be due to 
- ice, are described in my Geological Report for 1874 (p. 52), where an attempt is also made 
to account for them. It would appear that, in winter, the ponds freeze to the bottom, 
incorporating in the ice any boulders which may be there. The central, which is the 
deepest part of most of these ponds, would be the last to freeze, and any addition to the 
ice, as the cold increased, would be from water oozing in at this part. All the loss by 
evaporation, which must be great in this dry climate, would be on the upper surface 
and around the edges, while all the increase would be on its lower side, especially in the 
central part. Ponds receiving accessions of water from below, yet shallow enough to 
freeze to the bottom, have been observed on Hudson Bay to assume a distinct dome shape 
before spring. This tendency to grow from the centre, together with the expansion of 
the ice from the intense cold would have the effect of raising the boulders each year and 
Sec. III. 1886. 12. 
