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The important results so far obtained from this work, which is still in progress, may be summarized as follows:

1. Contrary to current belief,' there appears to be no such phenomenon as the direct reflection of an electron from a copper surface, or if the phenomenon exists at all, the number so reflected at potentials less than the ionizing potential is negligible.

2. The result obtained in (1) reveals, therefore, a method of determining with simplicity and with considerable accuracy the ionizing potential of a metal surface. (The careful determination of the values of such potentials is now being carried out.)

3. As the energy of electronic impact increases, the amount of reemission from copper increases up to about 300 volts, from which point on it remains essentially constant.

4. In the case of copper, the number of electrons detached per impinging electron is never found to exceed 1.3.

5. The number of secondaries per electron is somewhat increased by cleaning the surface by heat, and it appears to be somewhat decreased when the temperature of the surface is raised.

6. The maximum energy of emission of electrons released by electronic bombardment of copper increases from about 2 volts to about 5 volts as the energy of impact increases from 10 volts to 300 volts.

The full details of this work will be reported by one of us in a more extended article in the Physical Review.

1 von Baeyer, O., Verh. deut. physik. Ges., Berlin, 10, 1908 (903); also Gehrts, A., Ann. Physik., Leipzig, 36, 1911 (996). Horton and Davies, Proc. Roy. Soc., 97, 23, 1920, obtained results which they interpret in terms of reflections, but which we do not think demand such interpretation. The article by Hull, A. W., Proc. Inst. Radio Engineers, Feb., 1918, should also be read, though it does not bear upon this particular point.

CHRONOLOGY OF THE SAN JUAN AREA
BY EARL H. MORRIS

AMERICAN MUSEUM OF NATURAL HISTORY, NEW YORK

Communicated by H. F. Osborn, December 2, 1920

The chronological outline herein presented is the result of ten years' research among prehistoric aboriginal remains in that portion of the San Juan watershed lying in northwestern New Mexico, southwestern Colorado, and adjacent areas in Arizona and Utah. In the earlier stages of the work, financial support was given successively by the School of American Archaeology, the St. Louis branch of the Archaeological Institute of America, and the University of Colorado. For the past five years the work has been under the direction of the American Museum of Natural History as a part of the Archer M. Huntington Survey of the Southwest.

Approximately 250 archaeological sites have been visited and examined, 148 of them intensively. The distribution of the letter is as follows:

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For the present, the San Juan culture area may be considered coextensive with the watershed of the river whence it takes its name. This area is some 25,000 square miles in extent. It is a vast elevated plateau cut through by a few perpetual streams and numerous cliff-bordered washes which flow only during spring thaws or subsequent to heavy rains. Despite its general aspect of intense aridity, aboriginal culture flourished over the entire area to the extent that almost every locality capable of supporting an agricultural people is marked by ruins large or small. These remains belong to four major cultural horizons, in stratigraphic order as follows:

IV. Late Black-on-white

III. Early Black-on-white
II. Pre-Pueblo

I. Basket Maker

1. Basket Maker Period. The chief diagnostic characters of Basket Maker culture are: Pronounced dolichocephalic crania, without artificial occipital flattening; apparent total absence of permanent habitations; absence of bow and arrow; use of atlatl; high development of textile manufacture, with specialized types of sandals and burial baskets; absence of pottery; and the cultivation of one very primitive type of corn.

Remains of the Basket Maker period have been found only in a restricted area, all identified sites being within a radius of twenty-five miles of Bluff City, Utah. This region has been intensively worked by Dr. A. V. Kidder and Mr. Samuel H. Guernsey. Hence, to them belongs the credit for having identified and characterized this initial culture stratum.

2. Pre-Pueblo Period.-Crania from pre-Pueblo sites are for the most part dolichocephalic, like those of the Basket Makers; but among them. there are a minority of what appear to be brachycephalic crania with artificial occipital flattening, such as is typical of the subsequent periods.

Pre-Pueblo dwellings were flimsy, irregularly arranged, one-storied structures, with walls usually composed of poles set upright and plastered with mud. Occasionally, however, the basal portions of walls were faced with stone slabs set on edge, or more rarely, a wall was raised of crude masonry. Within the area of or adjacent to the buildings were circular subterranean or semi-subterranean chambers whose walls were formed by the sides of the excavated pits. These chambers were the prototype of the later kiva.

Perishable materials have not been recovered from this period. Pottery was made and commonly used, but both forms and ware were crude. Smooth ware predominated over corrugated, which is doubtfully present. On the average less than thirty per cent of the smooth ware bears painted decoration.

Pre-Pueblo remains have been found in every ruin-strewn locality where a search for them has been made. The sites occur singly and in groups, usually in open, unprotected places. As type sites may be mentioned those upon the mesas between the La Plata and Mancos rivers described elsewhere (Thirty-third Annual Report, Bureau of American Ethnology, Washington, 1919).

3. Early Black-on-white Period.-Crania from the early black-on-white period are all of apparent brachycephalic type with artificial occipital flattening. Dwellings of this period are more numerous and more widely distributed than those of earlier or later types. Both large and small structures occur, the latter being more common. Stone was the building

material generally used, but the individual blocks were never faced. In some cases the buildings may have been more than one story in height. The usual forms are rectangular, L or E-shaped. Circular subterranean chambers possessing all diagnostic kiva characters form a component part of every dwelling.

The textiles of this period are in every sense of Pueblo type. There is no remaining trace of the specialized Basket Maker bags, burial baskets, and square-toed sandals. At least three types of corn were cultivated. Ceramics was highly developed, both smooth and corrugated ware being very abundant. Certain forms of smooth ware, such as pitcher-shaped drinking vessels, and half-gourd ladles, and the absence of exterior decoration on bowls, are characteristic. Corrugated ware reached its highest development, as expressed by wide range of forms and extreme variety of decoration produced by the manipulation of the coils.

No early black-on-white site has been thoroughly excavated; hence, a type site remains to be described.

4. Late Black-on-white Period.-Crania from the late black-on-white period are, in superficial appearance, like those from the preceding period; that is, short from back to front with artificial occipital flattening.

Ruins belonging to this period are relatively few in number, but are

generally of large size. Typically, they are compact massive structures, two, three, and four or more stories in height, rising terrace fashion from an enclosed court. Stone was the customary building material. Many of the blocks were faced with admirable precision, and some of them were even polished. Kivas occurring as components of these structures are of large size, highly elaborate, and of unsurpassed construction. They are situated both within the enclosed courts and incorporated within the main building masses, the former subterranean, the latter above ground, and in some cases raised.

Specialization in architecture is much in evidence, several distinct local types, each distributed over a continuous minor area, being recognizable. The two prevailing localized forms are the cliff-houses, Mesa Verde as the type, and the great houses of the Aztec-Chaco region, the large ruin at Aztec, New Mexico, now nearing complete excavation by the writer, as the type specimen.

Cloth, basketry, and sandals are in every known respect identical with those of the preceding period. Smooth and corrugated ware continued through late black-on-white time; smooth ware reached the zenith of its perfection, while corrugated ware began to degenerate. Local specialization in pottery was a direct accompaniment of specialization in architecture. There are at least three major local types of pottery, namely, the Kayente, the Mesa Verde, and the Chaco, each with diagnostic forms and styles of ornamentation.

The following is a tentative interpretation of the data given above. In the Basket Maker period there is evidence of a dolichocephalic people, skilled makers of textiles, but ignorant of the utilitarian possibilities of clay and of the use of the bow and arrow. They were essentially nomadic, without permanent dwellings, but undergoing transition to sedentary life under the compelling influence of the cultivation of corn, of which they had a single primitive type.

In the pre-Pueblo period there may be postulated the blending of a brachycephalic element with the previous dolichocephalic stock. In this period a crude architecture was developed, the bow and arrow were introduced, and the manufacture of pottery begun.

In the early black-on-white period the brachycephalic stock seems entirely to have supplanted the dolichocephalic. Diagnostic Pueblo architecture expressed in fairly compact structures consisting of rectangular living rooms and circular ceremonial chambers, had taken definite form. Agriculture had been amplified by the addition of at least two types of corn and pottery making had been developed into a fine art.

In the culminating, or late black-on-white period, there are no evident changes in physical characteristics. Some potent integrating force, perhaps the pressure of predatory nomadic intruders, had brought together the hitherto widely scattered population into a few large and power

ful groups. These groups, in their several localities, carried Pueblo culture to its highest material perfection, at which point, for reasons as yet undetermined, Pueblo occupation of the San Juan area came to an abrupt close. As a terminating phase of late black-on-white time, there came a period of redistribution during which Pueblo culture was carried to the east, south and west.

The correlation of specific San Juan chronology with the chronology of the southwest as a whole is a work much of which is still to be done. The only other portion of the southwest for which a definite chronology has been proposed is that for the upper Rio Grande Valley in New Mexico. This area is both historic and prehistoric, in contrast to the area of the San Juan, which is exclusively prehistoric. Thus, the systematic series for the former presents a sequence, part of which can be historically dated, as established by Mr. N. C. Nelson and Dr. A. V. Kidder. The chronology of this Rio Grande area contains the following major divisions:

5. Modern Painted Ware

4. Historic Two-Color Glazed Ware

3. Three-Color Glazed and Painted Ware

2. Two-Color Glazed Ware

1. Two and Three-Color Ware

No. 1 of this series is comparable to IV in the San Juan series. Our data may, therefore, be interpreted as indicating a shift of cultures from the San Juan to the Rio Grande. Since ceramic types are but the horizon markers for the cultural epochs to which they belong, the chronologies for the San Juan and the Rio Grande areas may be combined and re-cast as in the following:

1. Basket Maker, or Initial Period

2. Pre-Pueblo, or Formative Period

3. Early Two-Color Painted Ware, or Period of Development

4. Late Two-Color Painted Ware, or Period of Culmination and Redistribution

5. Two-Color Glazed Ware

6. Three-Color Glazed and Painted Ware

7. Historic Two-Color Glazed Ware

It appears, therefore, that the establishment of a chronological scale for the area of the San Juan gives us a succession of cultural periods during the greater part of the prehistoric period for the southwest.

A full description of the sites examined, together with a formulation of the evidence upon which the preceding chronological sequence rests will be presented in future numbers of the Anthropological Papers of the American Museum of Natural History.

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