Alexander von Humboldt (1769-1859), the hugely influential German natural scientist visited Spain and the Canary Island as a prelude to his South American journey. Leaving France, he had hoped to join Bonaparte in Egypt, but could not find means of transport. Together, with botanist Aimé Bonpland, he found his way to Madrid where the unexpected patronage of the minister Don Mariano Luis de Urquijo convinced him to explore Spanish America instead. They sailed from La Coruña on June 5 1799 and stopped at Tenerife for six days. Here, Humboldt climbed El Teide, and his account of this ascent was to inspire many future naturalists, notably Charles Darwin, whose later planned stopover at Tenerife on the Beagle was frustrated by a cholera epidemic.

Personal Narrative of Travels to the Equinoctial Regions of America, During the Year 1799-1804 - Volume 1. Source http://www.gutenberg.org/dirs/etext04/qnct110.txt
We crossed Catalonia and the kingdom of Valencia, on our way to Madrid. We visited the ruins of Tarragona and those of ancient Saguntum; and from Barcelona we made an excursion to Montserrat, the lofty peaks of which are inhabited by hermits, and where the contrast between luxuriant vegetation and masses of naked and arid rocks, forms a landscape of a peculiar character. I employed myself in ascertaining by astronomical observations the position of several points important for the geography of Spain, and determined by means of the barometer the height of the central plain. I likewise made several observations on the inclination of the needle, and on the intensity of the magnetic forces.

On my arrival at Madrid I had reason to congratulate myself on the resolution I had formed of visiting the Peninsula. Baron de Forell, minister from the court of Saxony, treated me with a degree of kindness, of which I soon felt the value. He was well versed in mineralogy, and was full of zeal for every undertaking that promoted the progress of knowledge. He observed to me, that under the administration of an enlightened minister, Don Mariano Luis de Urquijo, I might hope to obtain permission to visit, at my own expense, the interior of Spanish America. After the disappointments I had suffered, I did not hesitate a moment to adopt this idea.

I was presented at the court of Aranjuez in March 1799 and the king received me graciously. I explained to him the motives which led me to undertake a voyage to the new world and the Philippine Islands, and I presented a memoir on the subject to the secretary of state. Senor de Urquijo supported my demand, and overcame every obstacle.

I obtained two passports, one from the first secretary of state, the other from the council of the Indies. Never had so extensive a permission been granted to any traveller, and never had any foreigner been honoured with more confidence on the part of the Spanish government.  
Many considerations might have induced us to prolong our abode in Spain. The abbe Cavanilles, no less remarkable for the variety of his attainments than his acute intelligence; M. Nee, who, together with M. Haenke, had, as botanist, made part of the expedition of Malaspina, and who had formed one of the greatest herbals ever seen in Europe; Don Casimir Ortega, the abbe Pourret, and the learned authors of the Flora of Peru, Messrs. Ruiz and Pavon, all opened to us without reserve their rich collections. We examined part of the plants of Mexico, discovered by Messrs. Sesse, Mocino, and Cervantes, whose drawings had been sent to the Museum of Natural History of Madrid. This great establishment, the direction of which was confided to Senor Clavijo, author of an elegant translation of the works of Buffon, offered us, it is true, no geological representation of the Cordilleras, but M. Proust, so well known by the great accuracy of his chemical labours, and a distinguished mineralogist, M. Hergen, gave us curious details on several mineral substances of America. It would have been useful to us to have employed a longer time in studying the productions of the countries which were to be the objects of our research, but our impatience to take advantage of the permission given us by the court was too great to suffer us to delay our departure. For a year past, I had experienced so many disappointments, that I could scarcely persuade myself that my most ardent wishes would be at length fulfilled.  

We left Madrid about the middle of May, crossed a part of Old Castile, the kingdoms of Leon and Galicia, and reached Corunna, whence we were to embark for Cuba. The winter having been protracted and severe, we enjoyed during the journey that mild temperature of the spring, which in so southern a latitude usually occurs during March and April. The snow still covered the lofty granitic tops of the Guadarama; but in the deep valleys of Galicia, which resemble the most picturesque spots of Switzerland and the Tyrol, cistuses loaded with flowers; and arborescent heaths clothed every rock. We quitted without regret the elevated plain of the two Castiles, which is everywhere devoid of vegetation, and where the severity of the winter's cold is followed by the overwhelming heat of summer. From the few observations I personally made, the interior of Spain forms a vast plain, elevated three hundred toises (five hundred and eighty-four metres) above the level of the ocean, is covered with secondary formations, grit-stone, gypsum, sal-gem, and the calcareous stone of Jura. The climate of the Castiles is much colder than that of Toulon and Genoa; its mean temperature scarcely rises to 15 degrees of the centigrade thermometer. 
We are astonished to find that, in the latitude of Calabria, hessaly, and Asia Minor, orange-trees do not flourish in the open air. The central elevated plain is encircled by a low and narrow zone, where the chamaerops, the date-tree, the sugar-cane, the banana, and a number of plants common to Spain and the north of Africa, vegetate on several spots, without suffering from the rigours of winter. From the 36th to 40th degrees of latitude, the medium temperature of this zone is from 17 to 20 degrees; and by a concurrence of circumstances, which it would be too long to explain, this favoured region has become the principal seat of industry and intellectual improvement.
When, in the kingdom of Valencia, we ascend from the shore of the Mediterranean towards the lofty plains of La Mancha and the Castiles, we seem to discern, far inland, from the lengthened declivities, the ancient coast of the Peninsula. This curious phenomenon recalls the traditions of the Samothracians, and other historical testimonies, according to which it is supposed that the irruption of the waters through the Dardanelles, augmenting the basin of the Mediterranean, rent and overflowed the southern part of Europe. If we admit that these traditions owe their origin, not to mere geological reveries, but to the remembrance of some ancient catastrophe, we may conceive the central elevated plain of Spain resisting the efforts of these great inundations, till the draining of the waters, by the straits formed between the pillars of Hercules, brought the Mediterranean progressively to its present level, lower Egypt emerging above its surface on the one side, and the fertile plains of Tarragona, Valencia, and Murcia, on the other. Everything that relates to the formation of that sea,* (* Some of the ancient geographers believed that the Mediterranean, swelled by the waters of the Euxine, the Palus Maeotis, the Caspian Sea, and the Sea of Aral, had broken the pillars of Hercules; others admitted that the irruption was made by the waters of the ocean. In the first of these hypotheses, the height of the land between the Black Sea and the Baltic, and between the ports of Cette and Bordeaux, determine the limit which the accumulation of the waters may have reached before the junction of the Black Sea,
the Mediterranean, and the Atlantic, as well to the north of the Dardanelles, as to the east of this strip of land which formerly joined Europe to Mauritania, and of which, in the time of Strabo, certain vestiges remained in the Islands of Juno and the Moon.)
which has had so powerful an influence on the first civilization of mankind, is highly interesting. We might suppose, that Spain, forming a promontory amidst the waves, was indebted for its preservation to the height of its land; but in order to give weight to these theoretic ideas, we must clear up the doubts that have arisen respecting the rupture of so many transverse dikes;--we must discuss the probability of the Mediterranean having been formerly divided into several separate basins, of which Sicily and the island of Candia appear to mark the ancient limits. We will not here risk the solution of these problems, but will satisfy ourselves in fixing attention on the striking contrast in the configuration of the land in the eastern and western extremities of Europe. Between the Baltic and the Black Sea, the ground is at present scarcely fifty toises above the level of the ocean, while the plain of La Mancha, if placed between the sources of the Niemen
and the Borysthenes, would figure as a group of  mountains of considerable height. If the causes, which may have changed the surface of our planet, be an interesting speculation, investigations of the phenomena, such as they offer themselves to the measures and observations of the naturalist, lead to far
greater certainty. 
From Astorga to Corunna, especially from Lugo, the mountains rise gradually. The secondary formations gently disappear, and are succeeded by the transition rocks, which indicate the proximity of primitive strata. We found considerable mountains composed of that
ancient grey stone which the mineralogists of the school of Freyberg name grauwakke, and grauwakkenschiefer. I do not know whether this formation, which is not frequent in the south of Europe, has hitherto been discovered in other parts of Spain.
Angular fragments of Lydian stone, scattered along the valleys, seemed to indicate that the transition schist is the basis of the strata of greywacke. Near Corunna even granitic ridges stretch as far as Cape Ortegal. These granites, which seem formerly to have been contiguous to those of Britanny and Cornwall, are perhaps the wrecks of a chain of mountains destroyed and sunk in the waves. Large and beautiful crystals of feldspar characterise this rock. Common tin ore is sometimes discovered there, but working the mines
is a laborious and unprofitable operation for the inhabitants of Galicia. 
The first secretary of state had recommended us very particularly to brigadier Don Raphael Clavijo, who was employed in forming new dock-yards at Corunna. He advised us to embark on board the sloop Pizarro,* (* According to the Spanish nomenclature, the Pizarro was a light frigate (fragata lijera).) which was to sail in company with the Alcudia, the packet-boat of the month of May, which, on account of the blockade, had been detained three weeks in the port.
Senor Clavijo ordered the necessary arrangements to be made on board the sloop for placing our instruments, and the captain of the Pizarro received orders to stop at Teneriffe, as long as we should
judge necessary to enable us to visit the port of rotava, and ascend the peak. 
We had yet ten days to wait before we embarked. During this interval, we employed ourselves in preparing the plants we had collected in the beautiful valleys of Galicia, which no naturalist had yet visited: we examined the fuci and the mollusca which the
north-west winds had cast with great profusion at the foot of the steep rock, on which the lighthouse of the Tower of Hercules is built. This edifice, called also the Iron Tower, was repaired in 1788. It is ninety-two feet high, its walls are four feet and a half thick, and its construction clearly proves that it was built by the Romans. An inscription discovered near its foundation, a copy of which M. Laborde obligingly gave me, informs us, that this pharos was constructed by Caius Sevius Lupus, architect of the city of Aqua Flavia (Chaves), and that it was dedicated to Mars. Why is
the Iron Tower called in the country by the name of Hercules? Was it built by the Romans on the ruins of a Greek or Phoenician edifice? Strabo, indeed, affirms that Galicia, the country of the Callaeci, had been peopled by Greek colonies. According to an
extract from the geography of Spain, by Asclepiades the Myrlaean, an ancient tradition stated that the companions of Hercules had settled in these countries.

The ports of Ferrol and Corunna both communicate with one bay, so that a vessel driven by bad weather towards the coast may anchor in either, according to the wind. This advantage is invaluable where
the sea is almost always tempestuous, as between capes Ortegal and Finisterre, which are the promontories Trileucum and Artabrum of
ancient geography. A narrow passage, flanked by perpendicular rocks of granite, leads to the extensive basin of Ferrol. No port in Europe has so extraordinary an anchorage, from its very inland
position. The narrow and tortuous passage by which vessels enter this port, has been opened, either by the irruption of the waves, or by the reiterated shocks of very violent earthquakes. In the New World, on the coasts of New Andalusia, the Laguna del Obispo
(Bishop's lake) is formed exactly like the port of Ferrol. The most curious geological phenomena are often repeated at immense distances on the surface of continents; and naturalists who have examined different parts of the globe, are struck with the extreme
resemblance observed in the rents on coasts, in the sinuosities of the valleys, in the aspect of the mountains, and in their distribution by groups. The accidental concurrence of the same causes must have everywhere produced the same effects; and amidst
the variety of nature, an analogy of structure and form is observed in the arrangement of inanimate matter, as well as in the internal organization of plants and of animals. 
Crossing from Corunna to Ferrol, over a shallow, near the White Signal, in the bay, which according to D'Anville is the Portus Magnus of the ancients, we made several experiments by means of a valved thermometrical sounding lead, on the temperature of the ocean, and on the decrement of caloric in the successive strata of water. The thermometer on the bank, and near the surface, was from 12.5 to 13.3 degrees centigrades, while in deep water it constantly
marked 15 or 15.3 degrees, the air being at 12.8 degrees. The celebrated Franklin and Mr. Jonathan Williams* (* Author of a work entitled "Thermometrical Navigation," published at Philadelphia.) were the first to invite the attention of naturalists to the phenomena of the temperature of the Atlantic over shoals, and in that zone of tepid and flowing waters which runs from the gulf of Mexico to the banks of Newfoundland and the northern coasts of
Europe. The observation, that the proximity of a sand-bank is indicated by a rapid descent of the temperature of the sea at its surface, is not only interesting to the naturalist, but may become also very important for the safety of navigators. The use of the thermometer ought certainly not to lead us to neglect the use of the lead; but experiments sufficiently prove, that variations of
temperature, sensible to the most imperfect  instruments, indicate danger long before the vessel reaches the shoals. In such cases, the frigidity of the water may induce the pilot to heave the lead in places where he thought himself in the most perfect safety. The waters which cover the shoals owe in a great measure the diminution of their temperature to their mixture with the lower strata of water, which rise towards the surface on the edge of the banks.
  The moment of leaving Europe for the first time is attended with a solemn feeling.