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to a given area and you cut down 100 of these trees containing the fluctu- ating series of rings just described and if you were to identify the outer- most ring in each of your 100 sections with the year 1957 and the next with 1956, and so on, you would find that when you got back to 1934 the ring would be small. I do not mean to imply that the 1934 ring would be exactly the same size in each of your 100 sections, but that there would be a tendency toward thinness. Continuing back in time, you would find the ring for 1913 to be small, 1904 small, 1902, 1899, and so on back. What you are encountering here is known as "cross-dating". In other words, the re is the same pattern of thick and thin rings in different trees over the same identical period of time. Thus, a definition of cross-dating would be absolute identification On a year-by-year basis of the same tree- ring pattern in a number of different trees. It is this phenomenon of cross- dating that is the fundamental reason for the success of the tree-ring method. Cross-dating makes possible the establislunent of an extended tree- ring series by overlapping a multitude of shorter series. The process works as follows: a modern tree is cut down, say one with a 100-year record, and the inner portion of this tree-ring series is matched or cross-dated with the outer portion of a 100-year specimen taken from an early pioneer's home. Then the outer portion of a beam from an early Spanish mission is matched against the inner portion of the "pioneer home" piece. Next, a beam from a late archaeological site is incorporated into the series extending the chronology backward in time. The process is continued by using tree-ring specimens from successively older and older archaeological sites. This method has been extraordinarily successful and we now have established a long tree-ring chronology for northern Arizona, parts of New Mexico, Colo- rado, and Utah that goes back to 59 B. C. It is easy to see then that with this master chronology at our disposal, a tree-ring specimen of unknown date can be compared with the master until the one place where cross-dating exists is found and the unknown specimen dated. Another and more direct method of establishing long- range tree-ring chronologies is to seek out ex- tremely long-lived trees. The late Dr. Edmund Schulman, of the Laboratory of Tree-Ring Research at the University of Arizona, engaged in such a pro- gram of research and was able to compile long-range chrortologies of this type in many areas of western America. His most spectacular success came with the discovery of the bristle-cone pines of California which yielded ex- ceptionally good records in excess of 4500 years. This brings us then to the "why" part of our question concerning the tree-ring method. In other words, why does the phenomenon of cross-dating exist? Let me emphasize at this point that tree growth is an enormously complex mechanism involving a whole host of variables, some internal, some external, and many of them, or perhaps, I should say. most of them, imperfectly understood as yet. I think it follows, however, since we have involved here a similar and simultaneous reaction on the part of the trees, that we must look for a single dominant, external factor that produces cross- dating. The Laboratory of Tree-Ring Research and other institutions have very conclusively shown that the single dominant factor causing cross- - 13 -