The direct outcome of my study, on the map of Costa Rica, of lines of equal earthquake effects, showed the maximum of the 1910 quake on the continental backbone, and the lines were crowded together along the western mountains. However, they spread out wider and wider along the Caribbean coastal plain, which is an elevated sea bottom on the northeast side of the country. In other words the terrific jolt was a deep slipping or scraping under the volcano line, and the elastic waves of like strong effects were close together in the mountains on the Pacific side, opposed by hard rock. On the other hand these waves, much feebler, widened out their lines in going through flat, soft strata on the Caribbean side. The answer seems to be that along the jagged rupture which underlies the volcanoes there is continuous upward pressure of lava, which occasionally is accelerated into a big bump or slip, now here, now there, as the whole great mountain range volcanically heaves through the ages. Our next journey was from Barrios across to Guatemala City, where we had distant views of such volcanoes as the pure cone of Agua and the sharp peak of Santa Maria, which in October of 1902 had blown out its flank and left a vast hole. The Guatemalan plateau of rich soil and abundant market products rises gradually from the wet banana lands on the Caribbean side to a height of 4,870 feet at Guatemala City. This is on the line of volcano cracks. Then the land plunges abruptly in a precipitous down-faulted slope, to a low flat shelf along the Pacific Ocean. This shelf is covered with the merging of many deltas formed by the streams and torrents which drain the well-watered plateau. Along this line at the top of the precipice is the chain of volcanoes, with rich coffee lands at their feet on the upper slopes. Coffee plantations were destroyed by steam, mud flood, and ash blasts in 1902, and similar destruction was destined to begin again in 1923. A large model of Central America has been built in a park in the open air in Guatemala City, showing magnificently the upland plateau and its mountains, the flat slope to the east, and the long straight steep plunge to the Pacific coastal shelf. This is one of the best illustrations of the block faulting of a continent, lifted like a huge flat slab along a crack, and tilted away from the Pacific. The Pacific block dropped down. The same structure is true, on a larger scale, of the line of the Andes, lifted as a volcano-covered slab, down-faulted along the Chilean coastal plain. The upland slopes away to the basin of the Amazon. In these studies we are experimenting with volcanoes on the scale of geography, but the principles involved apply to Mexico and to the Cascade Range in Oregon. They probably apply also to the Aleutian, the Kamchatkan, and the western Pacific arcs, considered as upheaved and eroded ridges. They are arcs because they are ancient calderas.