At no other horizon in the entire Stanley, excepting the tuff, is there so coarse a material.

Higher in the Stanley where the plant horizons were found, and in the Jackfork sandstones above, the vegetal remains occur in thin beds charged with small twigs and other small plant fragments, and also as scattered individual specimens. The latter are usually large, pieces of limbs and logs, but in all cases are macerated and eroded fragments-materials, it appears, which have been floated down stream from land areas to the south and southeast and out upon the delta to the north where they were engulfed in the sands.

Columbia University, June, 1920.

ART. VI.-Popocatepetl again in Activity; by PAUL WAITZ.

For some months we have seen from Mexico City on clear days of the rainy season that small eruption clouds were rising in puffs from Popocatepetl. This was a rather unusual spectacle, as since 1720, the year in which the last historically confirmed eruption occurred, the volcano apparently has shown only a very slight activity of fumaroles and solfataras. The long duration of this year's rainy season was not favorable for an investigation of the state of the mountain. As soon as better weather promised to allow of a successful ascension I gladly accepted the invitation of the Sociedad Cientifica "Antonio Alzate" to study the volcano. After careful preparation for the excursion I began the ascent in the company of some friends on the 10th of October of the present year; favored by wonderful weather conditions and good luck, the excursion had the very best of success.

We started from Amecameca (2532 m. above sea-level) on horseback at noon and after a five hours' ride through beautiful forests at sundown, reached Tlamacas, a locality situated on the north slope of the volcanic cone at a height of about 4000 m. above sea-level. The ranch house which years ago stood there at the side of a small hut used for sulphur smelting, had long since been destroyed by the revolution, but we found a few huts made of logs and covered with zacate-grass, which offered us a good shelter. After a rather uncomfortable night on account of the low temperature, we started on the 11th of October at 4 A. M. on horseback from Tlamacas and rode as far as Las Cruces (about 4500 m.) from which point the horses were sent back to Tlamacas. From here on we made use of a fairly good zigzag trail which had been constructed last winter by the sulphur diggers (azufreros): a proof that in the past winter no snow lay on the slope of the mountain, although in former years a cover of snow used to reach down to Las Cruces. While this cover of snow has disappeared almost entirely, the glacier of the volcano, which formerly on account of the thick cover of snow could not be observed at all, is still well preserved. This glacier lies in a depression between the main cone and a prominent promontory, the Pico del Fraile, and has been preserved up to the present date because the AM. JOUR. SCI.-FIFTH SERIES, VOL. I, No. 1.-JANUARY, 1921.

FIG. 1.-The crater of Popocatepetl on October 11, 1920. as seen from the lowest level of the crater rim. The west wall and the andesite porphyry in the background.

main cone is especially thick on this side and a very special kind of stratification diminishes the velocity of propagation of the heat waves from the depth toward the surface. The glacier reaches from the main summit, the Pico Mayor of the volcano (5450 m.), on the northwest slope of the cone, down to about 4800 m. We also found on the north side of the cone the side which was used for the ascent, small patches of snow in the depressions where it had been accumulated by the wind. On the east, south and west slopes the cone was entirely free from snow.

I arrived at the lowest part of the crater rim at 9:30 A. M. During the interval before my friends could join me I was able to study the crater and observe a rather strong eruption. At 11 o'clock we began to ascend on the rim of the crater, climbing along its east and south side in order to reach the highest point lying on the west side, where we arrived at 2 P. M. We climbed down on the other side of the crater rim until we again reached its lowest portion and from there went quickly down to Las Cruces and Tlamacas. Here we remained a second night and after having been able to observe and photograph a very strong eruption at 7 o'clock, the next morning we rode to Amecameca, and returned to Mexico City, where we arrived on the evening of the same day.

With the exception of the disappearance of the snow cover, the form of the mountain has not changed since the beginning of its new stage of activity. The crater also has preserved its form and figure, at least no considerable changes could be observed, and comparing the pictures of the crater which I made in 1905 with those taken on the present trip, I cannot find any changes. This does not apply, however, to the bottom of the crater. As long ago as 1895, Aguilera and Ordoñez, who at that time studied the mountain and measured the crater, remaining at its bottom for several days, described in its lowest portion a small lagoon, which was also well known to all those who had ascended Popocatepetl during the last 20 or 30 years. This lagoon has disappeared and in its place, but of much greater dimensions, I found an elliptical accumulation of andesite bowlders. This hill has a NW-SE longitudinal axis about 100 m. long, a transverse axis about 75 m. wide, and a height of 40 to 50 m. There is no doubt that this hill of black andesite bowlders is the upper part of a lava plug which after the