ries of Zumbalica, but I have found black obsidian with a conchoidal fracture in very large masses, immersed in bluish gray weathered perlite, amongst the blocks thrown out from Cotopaxi and lying near Mulalo. Of this, fragments are preserved in the Royal Collection of Minerals at Berlin. The pumice-stone quarries here described, at a distance of sixteen miles from the foot of Cotopaxi, appear therefore, to judge from their mineralogical nature, to be quite foreign to that mountain, and only to stand in the same relation to it, which all the volcanoes of Pasto and Quito, occupying many thousand square miles, present to the volcanic focus of the equatorial Cordilleras. Have these pumice-stones been the centre and interior of a proper crater of elevation, the external wall of which has been destroyed in the numerous convulsions which the surface of the earth has here undergone? or have they been deposited here upon fissures in apparent rest, during the most ancient foldings of the earth's crust? For the assumption of aqueous sedimentary alluvia, such as are often exhibited in volcanic tufaceous masses mixed with remains of plants and shells, is attended with still greater difficulties. The same questions are suggested by the great mass of pumice-stone, at a distance from all intumescent volcanic platforms, which I found on the Rio Mayo in the Cordillera of Pasto, between Mamendoy and the Cerro del Pulpito, 36 miles from the active volcano of Pasto. Leopold von Buch has also called attention to a similar perfectly isolated eruption of pumice-stone described by Meyen, which, consisting of boulders, forms a hill of 320 feet in height, near the village of Tollo, to the east of Valparaiso, in Chili. The volcano Maypo, which upheaves jurassic strata in its rise, is two full days' journey from this eruption of pumicestone. The Prussian Ambassador in Washington, Friedrich von Gerolt, to whom we are indebted for the first 35 "The volcano of Maypo (S. lat. 34° 15') which has never ejected pumice-stone, is at a distance of two days' journey from the ridge of Tollo, which is 320 feet in height and entirely composed of pumicestone, inclosing vitreous felspar, brown crystals of mica, and small fragments of obsidian. It is, therefore, an (independent) isolated eruption, quite at the foot of the Andes and close to the plain." Leop. de Buch, Desc. Phys. des Iles Canaries, 1836, p. 470. coloured geognostic map of Mexico, also mentions "a subterranean quarry of pumice-stone at Bauten," near Huichapa, 32 miles to the south-east of Queretaro, at a distance from all volcanoes 36. The geological explorer of the Caucasus, Abich, is inclined to believe from his own observations, that the vast eruption of pumice-stone near the village Tschegem, in the little Kabarda, on the northern declivity of the central chain of the Elburuz, is, as an effect of fissure, much older than the elevation of the very distant conical mountain just menioned. If, therefore, the volcanic activity of the earth, by radiation of heat into space during the diminution of its original temperature, and in the contraction of the superior cooling strata, produces fissures and wrinkles (fractures et rides), and therefore simultaneous sinking of the upper and upheaval of the lower parts", we must naturally regard, as the measure and evidence of this activity in the various regions of the earth, the number of recognizable volcanic platforms (open, conical, and dome-shaped mountains) upheaved upon fissures. This enumeration has been repeatedly and often very imperfectly attempted: eruptive hills 36 Federico de Gerolt, Cartas Geognosticas de los Principales Distritos Minerales de Mexico, 1827, p. 5. 37 On the solidification and formation of the crusts of the earth, see Cosmos, vol. i, pp. 164-166. The experiments of Bischof, Charles Deville, and Delesse have thrown a new light upon the folding of the body of the earth. See also the older, ingenious considerations of Babbage, on the occasion of his thermic explanation of the problem presented by the temple of Serapis to the north of Puzzuoli, in the Quarterly Journal of the Geological Society of London, vol. iii, 1847, p. 186; Charles Deville, Sur la Diminution de Densité dans les Roches en passant de l'état cristallin à l'état vitreux, in the Comptes rendus de l'Acad. des Sciences, t. xx, 1845, p. 1453; Delesse, Sur les Effets de la Fusion, t. xxv, 1847, p. 455; Louis Frapolli Sur la Caractère Géologique, in the Bull. de la Soc. Géol. de France, 2me série, t. iv, 1847, p. 627; and above all, Elie de Beaumont, in his important work, Notice sur les Systèmes de Montagnes, 1852, t. iii. The following three sections deserve the particular attention of geologists: Considérations sur les Soulèvements dús à une diminution lente et progressive du volume de la Terre, p. 1330; Sur l'Ecrasement Transversal nommé refoulement par Saussure, comme une des causes de l'élévation des Chaînes de Montagnes, pp. 1317, 1333, and 1346; Sur la Contraction que les Roches fondues éprouvent en cristallisant, tendant dès le commencement du refroidissement du Globe à rendre sa masse interne plus petite que la capacité de son enveloppe extérieure, p. 1235. and solfataras, belonging to one and the same system, have been referred to as distinct volcanoes. The magnitude of the space in the interior of continents which has hitherto remained closed to all scientific investigation, has not been so great an obstacle to the solidity of this work as is commonly supposed, as islands and regions near the coast are generally the principal seat of volcanoes. In a numerical investigation, which cannot be brought to a full conclusion in the present state of our knowledge, much is already gained when we attain to a result which is to be regarded as a lower limit, and when we can determine with great probability upon how many points the fluid interior of our earth has remained in active communication with the atmo sphere within the historical period. Such an activity usually manifests itself simultaneously in eruptions from volcanic platforms (conical mountains), in the increasing heat and inflammability of thermal springs and naphtha wells, and in the increased extent of circles of commotion, phenomena which all stand in intimate connection and in mutual dependence 38. Here again, also, Leopold von Buch has the great merit of having (in the supplements to the Physical Description of the Canary Islands) for the first time undertaken to bring the volcanic system of the whole earth, after the fundamental distinction of Central and Linear Volcanoes, under one cosmical point of view. My own more recent, and, probably for this reason, more complete enumeration, undertaken in accordance with principles which I have already indicated (pp. 245 and 271) and therefore excluding unopened bell-shaped mountains and mere eruptive cones, gives, as the probable lower numerical limit (nombre limite inférieur), a result which differs considerably from all pre 38 "The hot springs of Saragyn at the height of fully 5600 feet are remarkable for the part played by the carbonic acid gas which traverses them at the period of earthquakes. At this epoch, the gas, like the carbonated hydrogen of the peninsula of Apscheron, increases in volume and becomes heated, before and during the earthquakes in the plain of Ardebil. In the peninsula of Apscheron, the temperature rises 36°, until spontaneous inflammation occurs at the moment when and the spot where an igneous eruption takes place, which is always prognosticated by earthquakes in the provinces of Chemakhi and Apscheron." Abich, in the Mélanges Physiques et Chimiques, t. ii, 1855, pp. 364–365 (see Cosmos, vol. v, p. 175). r have vious ones. The question has been repeatedly raised whether in those parts of the earth's surface, in which the greatest number of volcanoes are crowded together, and the reaction of the interior of the earth upon the hard (solid) crust manifests the most activity, the fused part may not lie nearer to the surface? Whatever be the course adopted to determine the average thickness of the solid crust of the earth in its maximum: whether it be the purely mathematical one which is presented by theoretical astronomy, or the simpler course, founded upon the law of the increase of heat with depth and the temperature of fusion of rocks, still the solution of this problem presents a great number of values which are at present undetermined. Amongst these we 39 W. Hopkins, Researches on Physical Geology in the Phil. Transact for 1839, pt. ii, p. 311, for 1840, pt. i, p. 193, and for 1842, pt. i, p. 43; also with regard to the necessary relations of stability of the external surface; Theory of Volcanoes in the British Association Report for 1847, pp. 45-49. 40 Cosmos, vol. v. pp. 35-37; Naumann, Geognosie, Bd. i, pp. 66—76; Bischof, Wärmelehre, s. 382; Lyell, Principles of Geology, 1853, pp. 536 -547 and 562. In the very interesting and instructive work, Souvenirs d'un Naturaliste, by A. de Quatrefages, 1854, t. ii, p. 469, the upper limit of the fused liquid strata, is brought up to the small depth of 20 kilometres: "as most of the silicates fuse at 1231°.” "This low estimate," as Gustav Rose observes, "is founded in an error. The temperature of 2372°, which is given by Mitscherlich as the melting point of granite (Cosmos, vol. i, p. 26) is certainly the minimum that we can admit. I have repeatedly had granite placed in the hottest parts of a porcelain furnace, and it was always but imperfectly fused. The mica alone fuses with the felspar to form a vesicular glass; the quartz becomes opaque, but does not fuse. This is the case with all rocks which contain quartz; and this means may even be made use of for the detection of quartz in rocks, in which its quantity is so small that it cannot be discovered with the naked eye,-for example in the syenite of Plauen, and in the diorite, which we brought in 1829 from Alapajewsk in the Ural. All rocks which contain no quartz, or any other minerals so rich in silica as granite, such as basalt for example, fuse more readily than granite to form a perfect glass in the porcelain furnace; but not over the spirit lamp with a double current, which is nevertheless certainly capable of producing a temperature of 1231°." In Bischof's remarkable experiments, on the fusion of a globule of basalt, even this mineral appeared, from some hypothetical assumptions to require a temperature 264° higher than the melting point of copper. (Wärmelehre des Innern unsers Erdkörpers, s. 473). have to mention: the influence of an enormous pressure upon fusibility, the different conduction of heat by heterogeneous rocks, the remarkable enfeebling of conductibility with a great increase of temperature, treated of by Edward Forbes, the unequal depth of the oceanic basin, and the local accidents in the connection and nature of the fissures, which lead down to the fluid interior! If the greater vicinity of the upper limit of the fluid interior in particular regions of the earth may explain the frequency of volcanoes and the greater multiplicity of communication between the depths and the atmosphere, this vicinity again may depend either upon the relative average differences of elevation of the sea-bottom and the continents, or upon the unequal perpendicular depth at which the surface of the molten fluid mass occurs, in various geographical longitudes and latitudes. But where does such a surface commence ? Are there not intermediate degrees between perfect solidity and perfect mobility of the parts ?-states of transition which have frequently been referred to in the discussions relative to the plasticity of some Plutonic and volcanic rocks which have been elevated to the surface, and also with regard to the movement of glaciers. Such intermediate states abstract themselves from mathematical considerations, just as much as the condition of the so-called fluid interior under an enormous pressure. If it be not even very probable that the temperature everywhere continues to increase with the depth in arithmetical progression, local intermediate disturbances may also occur, for example, by subterranean basins (cavities in the hard mass), which are from time to time partially-filled from below with fluid lava and vapours resting upon it. Even the immortal author of the Protogea allows these cavities to play a part in the theory of the diminishing central heat :-"Postremo credibile est contrahentem se refrigeratione crustam bullas reliquisse, ingentes pro rei magnitudine id est sub vastis fornicibus cavitates."42 41 Cosmos, vol. v, p. 168. See also with regard to the unequal distribution of the icy soil, and the depth at which it commences, independently of geographical latitude, the remarkable observations of Captain Franklin, Erman, Kupffer, and especially of Middendorff (loc. cit. sup, s. 42, 47 and 167). 42 Leibnitz in the Protogæa, § 4. |