Page:Popular Science Monthly Volume 28.djvu/840

820 two or three points, but in a zone having borders parallel to the Pacific coast, with an average width of about thirty miles. This formation arises from the fact that Central America has had three successive shores, recording as many periods of least movement in the increase of the Cordillera, to each of which corresponds a line of contemporaneous volcanoes. The most ancient shore was of the Miocene period, when a system of trachytic and basaltic eruptions took place; then in the Pliocene rose the chain of the largest number of extinct volcanoes; while in the Quaternary and modern periods appeared the line of existing volcanoes and of others that have since become extinct. It is apparent, then, that the volcanic force has always been near the shore of the ocean, and has moved successively from the east to the west, so as to be at only a short distance from it, as the Cordilleras in their progressive elevation carried the shore farther in that direction. These views, incontestable to me, are plainly read on the strata of the country.

The system of volcanoes is completed by a chain of lakes alternating with them. The principal lakes are those of Managua and of the roads of Fonseca, the latter of which has been put in communication with the ocean by means of some volcanic convulsion. The roads of Nicoya and Chiriqui seem to me to be of the same origin. This part of the system is surely one of the most remarkable aggregations of lakes and volcanoes in the world, and strikingly reminds us, but on a grander scale, of that of the lakes of Limagne, Issoire, and Brassac, with the chain of the puys of Auvergne, which would correspond with the chain of the Marrabios. Starting at the roads of Fonseca, the chain of lakes and volcanoes continues, the former diminishing in importance, to San Salvador and Guatemala. I am not speaking of the numerous picturesque crater-lakes which we meet everywhere in Central America, and which I regard as an accident of no particular importance.

A phenomenon well worthy of attention may be observed at the foot of the chain of volcanoes near Ahuachapan, in San Salvador, in the Ausales, some three or four hundred conical tunnels scattered over a space of about three square leagues, their diameters varying from three or four metres to thirty or thirty-five metres, from which occur, at short intervals, eruptions of vapors, boiling water, and argillaceous mud of many colors. They are grouped by dozens very close together, and poison the plain with their acid and sulphurous emanations. The ground around them resounds under the feet of the traveler, but only along lines which seem to be immediately over the subterranean channels through which the hot water and gases circulate.

From this multiplicity of volcanoes it results that the ground presents a complicated net-work of ancient and modern lava-flows, crossing one another, volcanic alluvions, beds of cinders and tufas, "bad lands," and an extraordinary thermal activity. There also follows a remarkable frequency of earthquakes and subterranean noises, called