Passing to America, we come to interesting evidence of a very different class—I refer to the “big trees” (_Sequoia_) of California. Since these trees live in a semi-arid climate, the amount of rainfall is the chief factor in their growth, which finds an expression in the breadth of the annual rings measured on the stump of the tree when it is cut down. The method of utilizing the data was due to A. E. Douglass. A careful comparison was first made between the measurements of rings and the rainfall measured at neighbouring stations, and a formula was developed by which the rainfall of each year could be reconstructed from the tree-growth with a high degree of accuracy. In extrapolating to find the rainfall for earlier years before rainfall measurements began, various corrections had to be applied, for instance trees grow more rapidly when young than when they are old, while trees which are likely to live to a great age grow more slowly at first than trees which die younger. These methods were applied to nearly two thousand “big trees,” some of which were found to be four thousand years old, but it is pointed out that the corrections eliminate any progressive variation of climate which may have occurred, so that the results show only “cycles” of greater or lesser length. Summing up, Huntington says: “Judging from what we have seen of the rainfall of to-day and its relation to the growth of the Sequoias, high portions of their curve (of growth) seem to indicate periods when the winters were longer than now, when storms began earlier in the fall and lasted later into the spring, and when mid-winter was characterized by the great development of a cold continental high-pressure area, which pushed the storms of the prevailing zone of westerly winds far down into sub-tropical regions and thus caused sub-tropical conditions to invade what is now the zone of equatorial rains.” Neglecting later favourable periods, which are relatively short and unimportant, it is found that these conditions prevailed very markedly between 1200 B.C. and A.D. 200, with maxima about 1150 B.C., 700 B.C., and from 450 B.C. to 250 B.C.