Page:Climatic Cycles and Tree-Growth - 1919.djvu/75

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57
DETAILS OF CURVE PRODUCTION.

wood begins again with the year 1791 in a thick, rapid growth. The heartwood continues for some 20 years before changing again into the white sapwood, which persists to the outside. In order to make sure that this gap would not prevent satisfactory identification, a small portion was cut from another part of the outside of the tree, showing some 300 rings without interruption; but this additional piece I found in that case to be unnecessary.

In sections numbered 22 and 23, from the old Enterprise millsite, there are injuries which do not greatly alter the appearance of the rings, yet are sufficiently great to weaken the wood and cause it to break at several points. If the break in such case is across the rings, it is easy to carry the identity of rings past the injured point. But when the break in any wood "sample is all in one ring there may be a doubt as to whether the break is between two rings or in the middle of one. In the latter case there will apparently be an extra ring at that point. If the break is obviously between two complete rings, then an unknown number of rings may be lost at the broken point. The only way to carry the correct dating of the rings past such broken places is to secure samples from other parts of the same tree or from other trees, which show 100 to 200 rings on each side of the uncertain place without serious interruption. A simple cross-identification will show whether any rings are lost. However, in Nos. 22 and 23 just referred to, nearly all lines of breakage crossed the rings in a way that left no uncertainty. But No. 22 had an injury and a break between complete rings at about 1020 B. C. and a pronounced injury at about 1060 B. C. No. 23 had an extensive decayed place with the loss of about 35 rings at 1060 B. C. An extra piece cut from the stump of No. 23 carried the dating across these gaps with perfect satisfaction and in complete accord with No. 21 which had been secured 50 miles to the north.

Cross-identification between distant points.—The sequoias collected in 1915 had come from the immediate vicinity of Camp 6, about 7 miles east of Hume, and from Indian Basin, which is 3 to 4 miles north of Hume. The total extent of country covered was about 10 miles. All these were identified and found to be very similar in their characteristics. In 1918 the country represented was extended by sections from the new Camp 7, some 2 miles east of Camp 6. Nos. 20 and 21 were then obtained from the old Converse Hoist, 4 miles from Indian Basin and 15 miles from the Camp 7 district. Finally, 2 trees were obtained from the old Enterprise millsite, 50 miles from the other localities. It was realized at the time that there might be difficulties of cross-identification between these 2 trees at Enterprise and the other well-known and well-identified groups near Hume and the General Grant National Park. However, it was very gratifying to observe on close examination of these sections that no uncertainty was introduced in the identity of the rings. One realizes from this that, so far