Very compact trachyte, gray, Sulphur, seen only in the crater, where it is abundant Porous, granular trachyte, iron-stained, Soft, friable trachyte, yellowish, Fine-grained, decaying trachyte, with seams of flint, Granular trachyte, 66 66 (rare.) Thirty-five miles S.E. of Pichincha is the extinct volcano. of Antisana, loftier than Cotopaxi, and overtopped only by Chimborazo and Cayambi. Humboldt mentions three lava streams; we discovered a fourth on the north side, reaching down to Papallacta. The great stream called Volcan d'Ansango, can be traced for ten miles, and its thickness as determined by our barometer, is 500 feet, with an average slope of 15°. It consists mainly of a dark, tough, porphyroid trachyte in angular fragments. At the foot of the mountain near Padregal, is a plain containing innumerable rounded, symmetrical hills of volcanic earth. Half way between Antisana and Pichincha the steps of a horse give a hollow sound, showing that the rock is porous and perhaps cavernous. In the same plain is a hot spring (118°) exhaling sulphuretted hydrogen. We ascended Antisana, to the altitude of 16,000 feet, collecting in this ascent the following representative rocks: Cellular vitreous trachyte, black with few crystals, (common.) Fine grained, porphyroid trachyte, dark,* 66 66 reddish, Coarse Fine-grained Porous Fine-grained with augite crystals, 66 Fifteen miles S.W. of Antisana rises Cotopaxi, the loftiest and most symmetrical active volcano on the globe. Every representation, excepting the photograph taken by Farrand, is erroneous. Humboldt made the south slope 52°, and the north 50°. Guzman made the slope 69° 30! Villavicencio in his Geografia makes it 40°; and in a view drawn by Salas, the first artist in Quito, the slope is 35°. Spruce makes it 29° * This and the preceding are characteristic of the "lava streams." tom of the crater is covered with huge blocks of porphyry and trachyte scattered about in wild confusion. West of the center rises the real cone of eruption, an irregular heap of stones about 250 feet high and containing numerous fumeroles. As Moreno rightly says, all the vents are situated in this little cone. The present products are sulphur and plumose alum lining the fissures, and aqueous vapor, with a small percentage of carbonic and sulphurous gases. The temperature of the vapor just within the fumerole we found to be 184°, water boiling beside it at 189°-2. The gigantic wall which girdles this fiery mount, is not only lower on the west side, but a deep cleft leads down into the wilds of Esmeraldas. A year ago the column of smoke did not rise above the top of the crater, but the volcano has lately been showing signs of activity such as it has not exhibited since the last grand eruption of 1660. On the 19th of March, 1868, detonations were audible at Quito, five miles distant in a straight line; and three days after there were more thunderings, with a great column of vapor visible from Chillo, twelve miles to the east. These phenomena were accompanied by an unusual fall of rain. On the 16th of August, occurred the great earthquake, since which event Pichincha has not made any extraordinary display. The solid products of Pichincha since the Spanish invasion have been chiefly pumice and ashes. The roads leading to Quito cut through hills of. pumice. On the plain of Iñaquito and in the valley of Esmeraldas are vast erratic blocks of trachyte, some containing twenty-five cubic yards, and having sharp angles, and in some cases a polished unstriated surface. M. Visse does not consider them to have been thrown out of Pichincha, as LaCondamine and tradition have judged. We dislike to disagree with this habile observateur. It is true, as he says, that they could not have come out of the present cone at a less angle than 45°, for they would have hit the sides of the high escarpment and rolled back again, while at a higher angle they would not have reached their present location. But they could be the fragments of the upper portion of the original trachytic cone blown into the air, at the great eruption which cleared out the enormous crater. The following rocks we observed within and around Pichincha : Pumice, (lapilli,) Coarse-grained trachyte with augite crystals, Fine granular trachyte, reddish,t * The natives ascribe the earthquake of 1859 to Pichincha. Used as building material in Quito. Very compact trachyte, gray, (common.) 66 66 66 66 Sulphur, seen only in the crater, where it is abundant Porous, granular trachyte, iron-stained, Soft, friable trachyte, yellowish, Fine-grained, decaying trachyte, with seams of flint, Granular trachyte, 66 66 (rare.) Thirty-five miles S.E. of Pichincha is the extinct volcano of Antisana, loftier than Cotopaxi, and overtopped only by Chimborazo and Cayambi. Humboldt mentions three lava streams; we discovered a fourth on the north side, reaching down to Papallacta. The great stream called Volcan d'Ansango, can be traced for ten miles, and its thickness as determined by our barometer, is 500 feet, with an average slope of 15°. It consists mainly of a dark, tough, porphyroid trachyte in angular fragments. At the foot of the mountain near Padregal, is a plain containing innumerable rounded, symmetrical hills of volcanic earth. Half way between Antisana and Pichincha the steps of a horse give a hollow sound, showing that the rock is porous and perhaps cavernous. In the same plain is a hot spring (118°) exhaling sulphuretted hydrogen. We ascended Antisana, to the altitude of 16,000 feet, collecting in this ascent the following representative rocks : Cellular vitreous trachyte, black with few crystals, 66 66 reddish, Compact trachyte, gray, found at snow limit, Fine-grained" iron-stained, 66 dark, Fine-grained" with augite crystals, Fifteen miles S.W. of Antisana rises Cotopaxi, the loftiest and most symmetrical active volcano on the globe. Every representation, excepting the photograph taken by Farrand, is erroneous. Humboldt made the south slope 52°, and the north 50°. Guzman made the slope 69° 30! Villavicencio in his Geografia makes it 40°; and in a view drawn by Salas, the first artist in Quito, the slope is 35°. Spruce makes it 29° * This and the preceding are characteristic of the "lava streams." 30'. The true slope of the south side is 30° 45'; of the north, 26° 45'; and of the west and east a little over 30°. Spruce gives 121° as the apical angle of the cone; more correctly it is 122° 30'. On the summit is a circular parapet of scoriæ as on the Peak of Teneriffe. On the east side are signs of an ancient lateral eruption. Cotopaxi is emphatically the pumice-producing volcano. The ash and cinder, sand and pumice accumulations in the vicinity are immense. In one place (Quincherar) they are 600 feet deep. The new road to the capitol which crosses the Chisinchi ridge about half way between Cotopaxi and Iliniza, presents the following section showing the character and relative amount of material successively erupted: Soil, Fine yellow pumice, Compact black ashes with seams of pumice, Fine yellow pumice, Compact black ashes, Fine yellow pumice, Compact black ashes with seams of pumice. Near Tacunga is the following section : Soil, Stones and cinders, Fine pumice, Stones and cinders, Compact black ashes. The mud deposited at the foot of Cotopaxi, von Cotta would call a kind of volcanic tufa. The plain of Mulalo is strown with huge blocks of dark trachyte, some of them thirty feet square. They lie in rows; and Visse contends that they are not the product of volcanic eruption. The following are the rocks seen about Cotopaxi: Pumice, with dark specks of augite,* pure white, Porous trachyte, (common.) The pumice seen by Castelnau and Bates floating on the Amazons was doubtless brought down from Cotopaxi by the Pastassa. Very compact trachyte, dark gray, Granular trachyte, whitish with hematite scales, 66 66 gray, dark, very tough, black, Porous trachyte, iron-stained, Fine-grained trachyte, dark, augitic, 66 cellular, Soft, disintegrating trachyte, hardens under water, Coarse porphyroid trachyte, reddish, 66 66 light gray, Tunguragua is a beautiful cone rivaling Cotopaxi; but it is 2,000 feet lower. Spruce calls one of the slopes 43° 15', and the apical angle 92° 30'. We found the west, east and south slopes 38°, making an angle at the apex of 104°. The west side is covered with fine black sand; on the north is an immense stream of black porphyroid fragments, much resembling the Antisana currents.* At the base of the mountain there is a fine grained ferruginous sandstone; and at Guanandu there is a cliff presenting columns of dark trachyte having a distinct prismatic form. At Baños is a hot ferruginous spring (130°). The last eruption lasted from 1773 to 1780; but Spruce asserts that he saw smoke issuing from the western edge of the truncated apex in 1857. The lithology of Tunguragua is illustrated by the following specimens: Vitreous trachyte, black with occasional feldspar crystals, (common.) vesicular, Fine-grained trachyte, gray with augite crystals, Altar is the most alpine of the Ecuadorian mountains. From the west it appears to be what it undoubtedly is, a broken down volcano, presenting eight snowy needle peaks surrounding an immense crater. It has not been active since the days of the Incas; but there is a tradition (still living in its Indian name, capac-urcu, the chief,) that originally it over * Humboldt says that Antisana was the only Quito volcano where he saw anything like a lava-current. AM. JOUR. SCI.-SECOND SERIES, VOL. XLVII, No. 140.—MARCH, 1869. |