Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

ADDENDA. 627 and likewise to the generalization that the action of volcanoes, and the permanent elevation of the land (and consequently, as I believe, the ele- vation of mountain chains) are parts of the same phenomenon, and due to the same cause. Page 446. When I put together the few and exceedingly imperfect remarks on the subject of Miasmata, I did not know of Dr. Ferguson's remarkable disser- tation — the result of his investigations in Holland, Spain, Portugal, and the West Indies — on the Nature and History of Marsh Poison (Edin- burgh Royal Transact., vol. ix., p. 273). He there clearly proves the fact, which had struck me with so much surprise, namely, that the driest dis- tricts, which, according to common notions, would be considered as the most healthy, are often singularly the contrary. In his concluding re- marks. Dr. Ferguson says (p. 290), " One only condition, then,-33Pms to be indispensable to the production of the marsh poison, on all surfaces capable of absorption, and that is the paucity of water, where it has pre- viously or recently abounded. To this there is no exception in climates of high temperature ; and from thence we may justly infer, that the poi- son is produced at a highly advanced stage of the drying process^ And, from facts previously stated, it would appear that even in barren hilly coun- tries, the banks of mountain torrents, which had been overflowed, some- times became extremely insalubrious. In another place, Dr. Ferguson says, " It is from these (the dried and half-dried margins of lakes and marshes) that the poison uniformly emanates, and never from the body of the lake or pool ; and I think it may be fairly presumed, that water, as long as it can preserve the figure of its particles above the surface, is in- noxious, and that it must first be absorbed into the soil, and disappear from the eye, before it can produce any mischievous effects. Whoever in malarious countries waits for the evidence of putrefaction will, in all the most dangerous places, wai t too long, as every one can testify who has seen pestilence teem forth, to the paralyzation of armies, — from the bare barren sands of the Alentejo in Portugal, the arid burnt plains of Estremadura in Spain, and the recently flooded table-lands of Barbadoes." I cannot forbear quoting here a remarkable fact, mentioned by Humboldt, though interpreted in a different manner by that illustrious traveller. Speaking of the interm'*' oii fevers which are so common near the great cataracts, or raudales, of the" Orinoko, he says (^Pers. Nar., vol. v., p. 17) the causes " are violent heats, joined with the excessive humidity of the air, bad nu- triment, and, if we may beheve the natives (as well as the missionaries), the pestilent exhalations that arise from the bare rocks of the raudales." Further on (p. 83), he says, " many examples are adduced of persons, who