MADEIRA. 



633 



academy in 1820, and found the level of the Baltic 

 in calm weather several inches below them. He 

 also found the level of the waters several feet be- 

 low marks made seventy or a hundred years before. 

 He obtained similar results in other places. He 

 discovered deposits on the side of the Botbnian 

 Gulf, between Stockholm and Gefle, containing 

 fossil shells of the same species which now char- 

 acterize the brackish waters of that sea. These 

 occur at various elevations, of from one foot to a 

 hundred feet, and sometimes reach fifty miles in- 

 land. The result of this examination induced Mr 

 Lyell to declare his belief, that certain parts of 

 Sweden are undergoing a gradual rise to the amount 

 of two or three feet in a century. 



The relative changes of level of the sea and dry 

 land have been noticed in several other places, 

 especially in the neighbourhood of Pozzuoli, of 

 which M. Arago gave an account at the meeting of 

 the academy of sciences on the 15th of May, 1837, 

 together with the results of the observations of M. 

 Capocci, director of the observatory at Naples. 

 Mr Babbage has proposed an ingenious explanation 

 of these phenomena, viz. the local changes of tem- 

 perature in the lower strata of the earth ; and has 

 calculated that a change of 100 Fahrenheit that 

 should affect a formation of sandstone to a depth 

 of five miles, would cause an elevation at the sur- 

 face to the extent of twenty-five feet. 



The observations of Humboldt and Von Buch, 

 have led them to the opinion, that in submarine 

 eruptions, the strata previously forming the bottom 

 of the sea are uniformly elevated. Hence it is 

 easy to account for the presence of the limestone 

 in these islands, and the occurrence in it of fossil 

 shells and impressions. But in some, the limestone 

 is of more recent origin. 



The great lateral extension of many of the beds 

 of lava in these islands is one of the many evi- 

 dences of their having flowed under the pressure of 

 the ocean, as has been remarked in Iceland, the 

 Ferroe islands, and others whose submarine origin 

 is admitted by all observers. Where volcanic 

 action has broken out since the elevation, the 

 craters, or lateral openings and dykes, are every 

 where seen. In some of the beds of lava, the 

 vesicles or air bubbles are of prodigious size, two 

 or more feet in diameter, and those of smaller size 

 are every where met with, even in the hardest and 

 most compact lava, while that in which they are 

 so numerous as to give the lava the spongiform and 

 cellular aspect, occupies the sides of the hills or 

 covers large tracts of surface. The hills and moun- 

 tains with which the islands abound, marking the 

 sites of eruptions, and the plateaux composed of 

 compact lava and tuff, or covered by pumice and 

 scorified fragments, with blocks of obsidian, still 

 farther indicate a submarine origin, and render the 

 resemblance to Iceland and the islands already re- 

 ferred to still more striking. 



The interpositions of beds of scoriae and tuffa, 

 which are to be seen in the most distinct manner 

 alternating with the beds of basaltic lava all along 

 the coasts, in bold precipices, indicate a series of 

 ejections at successive intervals. In some of the 

 islands, high bluffs are composed entirely of very 

 hard, light -coloured tuffa, which, as at Fayal, might 

 be mistaken for sandstone; in other places detached 

 masses rise in pinnacles above the waters, or have 

 been cut through by the action of the waves, pre- 

 senting the most picturesque appearance. Some 

 of them are arched and the waters flow beneath. 



Great changes on the surface are going on at this 

 day. Within the recollection of many of the in- 

 habitants, torrents of water have rushed from the 

 mountains, carrying destruction in their course, 

 overwhelming cottages and cultivated spots with 

 huge masses of rocks, pumice, and loose earth. 

 There is one spot in St Michael's, several acres in 

 extent, which was verdant and cultivated, until on 

 a sudden the whole surface was covered with huge 

 fragments borne down in the course of one night, 

 and it is now a barren waste of loose rocks. In 

 another spot, on the side of a deep ravine, at the 

 bottom of which flowed a small stream, an enor- 

 mous mass of pumice, an acre or two in extent and 

 several hundred feet in thickness, suddenly sepa- 

 rated from the main body, which formed an exten- 

 sive plain, and fell into the ravine, leaving a yawn- 

 ing and frightful chasm. In Madeira these de- 

 structive effects have been more common and 

 terrific than in the Azores. The effect of a most 

 frightful flood, which occurred in 1803, has been 

 described by an eyewitness, when such a swelling 

 of the rivers took place, that they shortly rose and 

 overflowed their banks, rushing down the declivi- 

 ties, and sweeping away whole vineyards and plan- 

 tations, cattle, wine, stores, and the houses of the 

 inhabitants, who, with their whole families, perish- 

 ed. Rocks of enormous size, as well as the largest 

 trees, torn up by the roots, were carried from the 

 mountains to the sea. The vestiges of this dread- 

 ful flood are still to be seen. A crater, said to be 

 about three quarters of a mile in circumference, 

 had a segment of its larger circumference, with the 

 trees growing upon it, borne down into the bottom 

 of the crater. In the town of Funchal, whole 

 lines of houses with their inmates were swept into 

 the sea, churches, bridges, and edifices of every de- 

 scription were involved in the same general wreck. 

 It was computed that no less than three hundred 

 persons perished. 



The action of the waves is continually producing 

 changes along all the coasts, the interposed beds of 

 tuff and cinders are washed away, and the compact 

 beds of lava being left unsupported, fall. The 

 shores are strewed with huge blocks and fragments 

 of all sizes. 



Did not the geological structure of the islands 

 furnish abundant evidence of volcanic action, we 

 find it on record in the islands, and in one of the 

 earliest works, that eruptions and earthquakes 

 have occurred at various times, as well within 

 the recollection of living witnesses as at earlier 

 periods. 



The first earthquake of which we have been able 

 to find any record was on the 25th of October, in 

 the year 1522, when five thousand persons were 

 destroyed. It occurred at daylight, and was par- 

 ticularly violent in the district of Fanaesda Ajuda 

 and Maia, on the north coast of St Michael's. It 

 extended to a considerable distance, nearly destroy- 

 ing the large city and port of Villa Franca. A 

 large part of the town was laid in ruins, and the 

 Franciscan convent and other buildings were over- 

 whelmed by a torrent of ashes and mud which has 

 since become consolidated into a compact tuffa. 

 Although the destruction of this place is now al- 

 ways attributed, by the present inhabitants, to an 

 earthquake, it is beyond doubt than an eruption 

 took place, of which we find traces in the compact 

 lava covering the tuff and running out into the sea, 

 forming indeed a wall with points here and there 

 appearing above water, and resembling somewhat 



