The following description of the volcano Irazu as it appeared in May 1920 may serve to give a general setting for both the geology and geography of the region.

THE VOLCANO IRAZU IN 1920

Though the central range of Costa Rica is of volcanic origin, only two volcanoes are active. Poas has been active throughout historic time, but Irazu was dormant for nearly 200 years and resumed activity in 1917. Poas has been described many times but Irazu seems to have received scant attention, since it resumed activity. Irazu is easily reached from the Atlantic side. The town of Cartago is a fivehour ride by train from Limon, and the summit of Irazu is a four-hour ride on horseback from Cartago.

Its summit is about seven miles north of and 6425 feet higher than Cartago, and 11,325 feet above sea level. It is the highest mountain in central Costa Rica, but Turrialba, six miles east, is only 360 feet lower.

The photographs in this article show the present condition of Irazu better than it can be described. Vapor rises about 1000 feet and spreads out in a great fan. The eruptions are not always steady, but sometimes come in puffs that rise high above normal, and on some days the amount of vapor is much less than on others. The guide told the writer that the amount was below normal on the day when the photographs were made. More than a mile from the crater one can hear a dull roar, which grows much louder on the days when the great amount of vapor is ejected.

Viewed from Cartago the vapor seems to rise four or five hundred feet above the top of the peak and to vary considerably in volume. It rises until it strikes the westerly moving air currents, when it flattens out and moves westward.

From Cartago the mountain appears as the summit of a gently curving surface rising from the main mountain range of Costa Rica (see figure 23). Though it rises nearly 6,500 feet above Cartago, the distance seems much less even to the experienced mountaineer.

The costal plains of the Santa Clara region lie north of Irazu, and viewed from there, Irazu appears as a jagged peak in a jagged mountain pass, though it rises only a little above the main range. It rises 11,000 feet in 10 miles from the Santa Clara plain.

From Cartago the writer rode to the top of Irazu in four hours and had his horse been a good one the trip could have been made in less time. The average grade of the trail is 20 per cent for most of the way, but near the summit is nearly 30 per cent in some places. In places the trail has been worn and washed out to 15 feet below the surface, and the sides are made up of volcanic ash in almost vertical walls.

The top of the mountain borders the crater on the south, and the usual path of ascent brings one to the south summit so that he may look down into the crater from the best viewpoint.

The accompanying photographs and topographic maps show the present condition of the mountain. To the west and north steep ash ridges limit the crater, while to the east and south the main mountain mass rises steeply from the crater floor.

Sapper's contour map of the crater published in 1902 compared with the

writer's map shows large changes since he made his observations in 1893-98. Sapper20 shows 10 or 11 small craters, while only three were seen by the writer.

Sapper's map is reproduced for the sake of comparison (see figure 26). The writer's map was made from photographs and notes (see figure 25). He made 28 exposures from various parts of the crater and crater rim. Sapper's II and g seem about the same now as in 1900 and H has undergone little change. I has become the site of a small lake, or at any rate a small lake occupies about the same situation as I. If the other craters are still extant they are not visible on account of the vapors in the main crater. The flat, ash-covered plain, I, has increased greatly in width or has disappeared. Sapper shows this plain sloping 120 feet in % of a mile. Figure 17 shows the flatness of a plain that occupies part of the same area, at the present time, but it seems likely that I no longer exists. Figure 20 shows the present plain in relation to the hill west of the crater. The plain is 200 or 300 feet below that hill, while Sapper's map shows I on the same level as the hill. Sapper's K seems to have been the highest plain inside the walls of the crater in 1920. In figure 20, taken as the writer interprets it, from plain K, crater g is not visible, while the hill to the west is high above the plain. A ridge of dust west of the crater has increased considerably in height since 1900. Calvert's21 observations in 1910 show the condition little changed from 1898.

The cloud of vapor which arises from the crater contains a great deal of dust and occasionally the dust settles several miles from the mountain. Pumice and basalt bombs are abundant in the older dust deposits., Where little streams have cut valleys they show coarse cinders interbedded with ash and dust. Some basalt flows are not deeply covered with dust. The crater floor is about 3,000 feet wide and the inner mass seems to have fallen in when the mountain stood higher than at present. The top of a basalt flow is about 300 feet above the broad floor of the crater on the south and is sharply truncated at the crater edge (see figure 19 near upper righthand corner).

To the north Irazu is very steep. Six miles from the summit the Toro Amarillo River is 1,500 feet above sea level and its valley at this place is about 500 feet deep. For 3,000 feet the south slope is much the same as the north; thence it decreases in steepness and continues gentler to the foot of the mountain at 4,700 feet in the Cartago region. The east slope is not as steep as the north and the descent is only about 2,500 feet in two and one-half miles, where the mountain joins Turrialba. On the northwest the volcanic range continues and Irazu joins Barba in a pass about 5,000 feet high through which the Limon-San Jose railroad used to pass.

The western margin of the crater is a steep-sided volcanic dust ridge and the south side makes up the main part of the mountain, the highest part of which is at the crater margin. The writer did not examine the north and west sides of the crater rim, but the north side seems to be an ash ridge like the west, while the east side is extensive like the south. Pittier22 mentions an old crater about one kilometer southeast of the top and Sapper tells of another east of the east top.

20 Sapper, Karl, Die mittelamerikanischen Vulkane: Petermann's Mitt. Erg. 38, no. 178, p. 117, 1913.

21 Calvert, A. S. and P.P., A Year of Costa Rican Natural History, p. 131, 1917. 22 Loc. cit., p. 118.