AND AMERICAN RURAL SPORTS. 



225 



the transporting power of running water, to consider all the 

 numerous agents which may co-operate in the lapse of ages, 

 in conveying the wreck of mountains to the sea. A gra- 

 nite block might remain stationary for ages, and defy the 

 power of a large river; till at length a small spring may 

 break out, surcharged with carbonic acid, — the rock may 

 be decomposed, and a streamlet may transport the whole 

 mass to the ocean. 



The subtraction of many of the elements of rocks by the 

 solvent power of carbonic acid, ascending both in a gaseous 

 state and mixed with spring-water in the crevices of rocks, 

 must be one of the most powerful sources of those internal 

 changes and re-arrangement of particles so often observed 

 in strata of every age. The calcareous matter, for example, 

 of shells, is often entirely removed and replaced by carbo- 

 nate of iron, pyrites, or silex, or some other ingredient, such 

 as mineral waters usually contain in solution. It rarely 

 happens, except in limestone rocks, that the carbonic acid 

 can dissolve all the constituent parts of the mass; and for 

 this reason, probably, calcareous rocks are almost the only 

 ones in which great caverns and long winding passages are 

 found. The grottos and subterranean passages, in certain 

 lava-currents, are due to a different cause, and will be spo- 

 ken of in another place. Ly ell's Geology. 



ERUPTION OF JORULLO IN 1759. 



As another example of the stupendous scale of modern 

 volcanic eruptions, we may mention that of Jorullo, in 

 Mexico, in 1759. We have already described the great 

 region to which this mountain belongs. The plain of Mal- 

 pais forms part of an elevated plateau, between two and 

 three thousand feet above the level of the sea, and is bound- 

 ed by hills composed of basalt, trachyte, and volcanic tuff, 

 clearly indicating that the country had previously, though 

 probably at a remote period, been the theatre of igneous 

 action. From the era of the discovery of the New World 

 to the middle of the last century, the district had remained 

 undisturbed, and the space, now the site of the volcano, 

 which is thirty-six leagues distant from the nearest sea, was 

 occupied by fertile fields of sugar-cane and indigo, and wa- 

 tered by the two brooks Cuitimba and San Pedro. In the 

 month of June, 1759, hollow sounds of an alarming nature 

 were heard, and earthquakes succeeded each other for two 

 months, until, in September, flames issued from the ground, 

 and fragments of burning rocks were thrown to prodigious 

 heights. Six volcanic cones, composed of scoriae and frag- 

 mentary lava, were formed on the line of a chasm which 

 ran in the direction from N.N.E. to S.S.W. The least of 

 3 L 



these cones was three hundred feet in height, and Jorullo, 

 the central volcano, was elevated one thousand six hundred 

 feet above the level of the plain. It sent forth great streams 

 of basaltic lava, containing included fragments of primitive 

 rocks, and its ejections did not cease till the month of Fe- 

 bruary, 1760. Humboldt visited the country twenty years 

 after the occurrence, and was informed by the Indians, 

 that when they returned long after the catastrophe to the 

 plain, they found the ground uninhabitable from the exces- 

 sive heat. When the Prussian traveller himself visited the 

 locality, there appeared, round the base of the cones, and 

 spreading from them as from a centre over an extent of four 

 square miles, a mass of matter five hundred and fifty feet in 

 height, in a convex form, gradually sloping in all directions 

 towards the plain. This mass was still in a heated state, 

 the temperature in the fissures being sufficient to light a 

 cigar at the depth of a few inches. On this convex protu- 

 berance were thousands of flattish conical mounds, from six 

 to nine feet high, which, as well as large fissures traversing 

 the plain, acted as fumeroles, giving out clouds of sulphuric 

 acid and hot aqueous vapour. The two small rivers before 

 mentioned disappeared during the eruption, losing them- 

 selves below the eastern extremity of the plain, and re-ap- 

 pearing as hot springs at its western limit. Humboldt 

 attributed the convexity of the plain to inflation from below, 

 supposing the ground, for four square miles in extent, to 

 have risen up in the shape of a bladder, to the elevation of 

 five hundred and fifty feet above the plain in the highest 

 part. But this theory, which is entirely unsupported by 

 analogy, is by no means borne out by the facts described; 

 and it is the more necessary to scrutinize closely the proofs 

 relied on, because the opinion of Humboldt appears to have 

 been received as if founded on direct observation, and has 

 been made the groundwork of other bold and extraordinary 

 theories. Mr. Scrope has suggested that the phenomena 

 may be accounted for far more naturall}', by supposing that 

 lava flowing simultaneously from the different orifices, and 

 principally from Jorullo, united into a sort of pool or lake. 

 As they were poured forth on a surface previously flat, they 

 would, if their liquidity was not very great, remain thickest 

 and deepest near their source, and diminish in bulk from 

 thence towards the limits of the space which they covered. 

 Fresh supplies were probably emitted successively during 

 the course of an eruption which lasted a year, and some of 

 these resting on those first emitted, might only spread to a 

 small distance from the foot of the cone, where they would 

 necessarily accumulate to a great height. 



The showers, also, of loose and pulverulent matter from 

 the six craters, and principally from Jorullo, would be com- 

 posed of heavier and more bulky particles near the cones, 

 and would raise the ground at their base, where, mixing 



