VOLCANO POPOCATEPETL AND ITS VICINITY. 53 



west side, nor do I recollect to have seen one, the east and west sides of which 

 had the same elevation. But this singular law holds good not alone for volcanic 

 cones. 



On the plain of Perote, near a silver mine called " La Preciosa," there are three 

 small lakes of nearly circular form, which are evidently of volcanic origin. The 

 one nearest to the mine is the largest ; its longest diameter runs from N. 31° E. to S, 

 31° W., and measures nearly 2000 yards. The surface of the water is about twenty- 

 five feet lower than that of the surrounding plain, and tlie banks are perpendicular. 

 The Director of the mine informed me that he had attempted to sound the lake, but 

 found no bottom at 250 fathoms. The water is brackish, and its temperature 

 was 69°. 2, while the temperature of the air in shade was 65°.0 (noon, Sept. 20, 1856). 

 The two other lakes are of similar form, but somewhat smaller. No one of them 

 has an outlet, nor do any streams empty into them, as it does not rain more than 

 four or five times a year in their vicinity. Xeverthcless the water is perfectly 

 clear, and of the dark color of sea-water in deep soundings. The shores of these 

 three lakes on the east side, are not elevated above the general level of the sur- 

 rounding plain, but on the west side they rise into hills (the one at the larger lake 

 being about 160 feet high) sloping gently towards the plain, but falling off abruptlv 

 towards the lake, presenting the appearance of an excavation, of which the material 

 was thrown up on its western banks. The surface of the hill consists of coarse 

 sand. The trade-winds, which are prevalent in these regions and blow from E. 

 N. E., may, perhaps, furnish an explanation of this peculiar elevation of the western 

 walls of the more recent craters above their eastern rims. 



As stated before, no lava is seen at the surface of Popocatepetl on its northern 

 and eastern slopes; but at rare intervals, in the deeper ravines (barrancas), solid 

 ledges of very compact lava are to be found. Passing around the volcano from the 

 north through east to the south, the first lava-stream on the surface is met with on 

 the eastern slope of the cone, commencing a little below the limit of perpetual 

 snow ; this stream has probably its origin at or near the present crater, but the 

 upper part of it is covered by volcanic sand and snow. It is soon joined by another 

 and larger stream, the outlines of whicli arc distinctly defined above the snow-mantle 

 of the cone up to the edge of the crater, where it forms a point projecting a little 

 above the general profile of the mountain, wlicn seen from the north. The 

 altitude of this point was determined trigonometrically, and in the following ob- 

 servations it is distinguished by the name of the "eastern peak," from tlie higlicst 

 or "western peak." The stream is called "el espinazo del diablo" (tlie devil's 

 backbone), and extends about three miles down, losing itself near the eastern base 

 of the mountain. 



On the southeastern and southern slopes large lava-streams extend from the 

 crater down to the edge of the plateau (from whicli the volcano rises ten thousand 

 feet vertically), and cover a space of many square miles. The surflice of the 

 lower and nearly horizontal parts of these lava-fields (pedregals) is studded witli 

 boulders of all dimensions, mostly approaching to a globular form, or to segments 

 of globes, and containing much iron and nickel. The lava of these fields is 

 sometimes of a red (cinnabar) or brownish, but more generally of a dark gray, 



