the balance of 37 units is made to stretch itself over 199 days, a brief, intensely hot summer would be followed by a very long and cold winter, and as this condition lasted for many centuries, it seemed sufficient to produce a glacial epoch." It would be going, too far, however, to assume that these conditions necessarily produced glacial periods whenever they occurred, and Ball himself points out that even on astronomical grounds, several conditions must concur before high eccentricity alone would affect climate. But even with this reservation the same objection applies to assigning, this as the sole or principal cause of Ice Ages, as to precession alone, viz. that periods of high eccentricity occur too frequently to allow us to suppose that every such period in the past has had its corresponding glacial period. There was a _maximum_ phase of eccentricity 700,000 years ago, even higher than that of 210,000 years, and there must have been at least two or three such _maxima_ within each of the twenty-eight geological ages. But there are only two or three traces of glacial periods in past epochs on which geologists can rely with confidence, as proving extensive ice-action--one in the Permian, the other in the Carboniferous age. There are a few other instances which look like glacial action, as the conglomerate of the Superga at Turin, the Flysch of Switzerland, the great conglomerate at the base of the Devonian; and Professor Geikie thinks that the oldest Cambrian rocks in the West Highlands have been rounded and smoothed by ice before the Silurian strata were deposited on them. But even if these were authenticated and proved to be due to general and not merely local causes, they would not supply anything like the number of glacial periods required by Croll's theory. Croll attempts to meet this by the extensive denudation which has repeatedly carried away such large portions of land surface; but this scarcely explains the absence of the boulders of hard rocks, which accompany every moraine and iceberg; and still less the continuance of the same fauna and flora throughout whole geological periods with little or no change. We have no such abrupt changes as during the last glacial period, when at one time the canary laurel flourished in Central France, while at another the reindeer moss and Arctic willow extended to the Pyrenees, both occurring within what may be called a short time, geologically speaking. On the contrary, there seems to have been no material changes in the flora throughout very long geological periods such as that of the Coal Measures.