Stuebel (Dr Alphons)

Through a simple oversight, no reference was made to the article written by Father Ernst Schmitz about Dr Alphons Stuebel when we dealt with the geology of Madeira. To what was said there must be added the authoritative opinion of that illustrious German naturalist, together with the no less judicious remarks, which accompany the said article, of the former professor of natural sciences at our Diocesan seminary (Seminário).

There is a wide divergence of opinion, at least on secondary questions, among the geologists who in the course of this century have studied the formation of the island of Madeira most closely. But just as the collision of bodies and their mutual friction often produce a spark and light, so it may be hoped that more light will also be shed on this question as competent men give their opinions after conscientious study, even if at first they reach differing results.

The opinions put forward by Luiz da S. Mousinho Albuquerque, by Dr J. Macaulay, Sir Charles Lyell, Dr Oswald Heer of Zurich and above all by Dr G. Hartung, who treated the subject more fully than any of them in his book Madeira & Porto Santo, etc., Leipzig, Engelmann, 1864, are fairly well known in Portugal. But very few of those interested in the geology of Portuguese lands will be aware of the painstaking and conscientious studies that Dr Alphons Stuebel of Dresden made of Madeira. He was in Madeira in 1862 and 1863, and after detailed and prolonged observation and exploration of the terrain he built the geological map of Madeira – not on a sheet of paper, but in relief, in modelling paste. On it every height and hollow, extinct crater, stream, levada (irrigation channel), settlement, etc., is shown with the greatest clarity, all on a scale of 1 to 55,000, so that it is more than a metre long, with a corresponding width.

This unique map of Madeira is in the Funchal Seminary Museum (Museu do Seminário do Funchal), and it admirably combines the gifts of science and art, all of which aim to facilitate and assist the study of geology, and of volcanology in particular.

Surely no little weight must be given to the opinion of a man whose whole life was devoted to volcanology, and whose ample fortune allowed him to go and study de visu the most interesting volcanic regions of our globe. We therefore believe we are doing a service in making better known this opinion of Dr Stuebel, which is set out in broad outline in his monumental work Die Vulkanberge von Ecuador and in the part of it Ueber des Wesen des Vulkanismus, published separately in Berlin, A. Asher & Co., 1897.

An example of the close union of two monogenetic volcanic elevations (that is, elevations due to a single volcanic cause, even if it manifests itself at long intervals of time) is provided, on a far larger scale than Pichincha on the Equator, by the island of Madeira. The whole mass of its mountains, which over a length of nearly 60 kilometres and a width of 15 to 20 rises to an altitude of 1,800 metres, was simply the product of two eruptive centres. And both of them, as can be concluded with great certainty from the way in which they are joined, were active at the same time.

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The larger and higher of these two monogenetic structures is a caldera-shaped elevation that occupies more than two-thirds of the island. The caldera proper, which lies almost at the centre of the whole mass, is nearly 4 1/2 kilometres in diameter and 1,200 metres deep. A long valley (the former crater), which becomes very narrow in its lowest part and turns into a ravine, and which falls 600 metres over a length of 11 kilometres, drains the waters of this caldera, called Curral das Freiras (‘nuns’ fold’). Among the many peaks surrounding the caldera, the highest, and indeed the highest point on the island, is Pico Ruivo (1850) metres. From the rim of the caldera immense ridges stretch out in every direction, and although these differ greatly in length, structure and shape, the observer is left with the impression that this main part of the island is, in broad outline, a symmetrical structure produced by volcanic forces. ‘Very different is the second mass of the island, much smaller in extent, but only 300 metres lower than the first. It is a dome-shaped elevation, flattened at the top, where one would expect a crater; it forms a plateau more than 3 kilometres in diameter, called the Paul da Serra (‘upland marsh’).

The two eruptive centres, with their piled-up masses, lie so close to each other that their rocks not only touch but, to a certain extent, interlock. The boundary between the two masses is still clearly visible, marked by two very deep valleys running in opposite directions: one to the north, that of São Vicente, and the other to the south, that of Ribeira Brava (‘wild river’). The headwater regions of these valleys, which erosion has widened and transformed into calderas with several branches, are separated only by a relatively very thin but high wall, a kind of crest bristling with peaks, which connects the plateau of the Paul da Serra with the western wall of the Curral das Freiras caldera.

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Despite all the resemblance that the valleys of São Vicente and Ribeira Brava, with their caldera-shaped widenings, bear to the great caldera of Curral das Freiras, their significance for the geological structure of the island is essentially different. They are merely inter-hill spaces widened by erosion, whereas the valley of Curral das Freiras constitutes a true crater, and the stream of Curral, or of Socorridos, opens the way to this true eruptive centre.

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We insist on this necessary distinction concerning the formation of these valleys precisely because Sir Charles Lyell, in his description of the island of Madeira (Elements of Geology), failed to make it.

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While these two eruptive centres were piling up almost the entire mass of the island as it rose above sea level, it seems that some secondary centres, of which visible traces remain, were also active. This is evident above all on the high, steep northern coasts. In particular, we regard the Penha de Águia as an important remnant of an ancient destroyed crater.

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In addition, the island, whose volcanic activity died out long ago, possesses a number of well-preserved crater formations, although they are small. Their position and their connection with the main mass of the island make it more than probable that the source of their scanty eruptive material must be placed within the mass of the island itself, and not in the immense depths beneath it from which the island itself took its origin. We may mention, for example, the shallow crater of the pond (lagoa) of the Serra de Santo António at the far eastern end of the island, as well as the small eruptive cones and lava flows that form the base of the plateau above Porto Moniz (‘Moniz’s harbour’) at the far north-western end of the island.

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We do not wish to go into detail here about the tectonic structure of the volcanic mountains and their ramifications, but we should like to point out that the gradients of the mountain slopes and of the lombos show angles of inclination almost identical to those of the volcanic mountains of the Equator. As there, the spurs that radiate in all directions from the central mass remain fairly horizontal in their upper part, but descend in very steep slopes as they approach their bases.

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This topographical and genetic description of Madeira, merely sketched here, is very well represented on the map that Jakob Melchior Ziegler drew on the basis of his own and others’ studies and drawings, and also on a relief map deposited at the Geological Institute of the University of Berlin.

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All our theories on the nature of volcanism were first suggested to us by the geological examination of the island of Madeira, and soon afterwards confirmed by that of the Cape Verde islands. But we did not wish to publish them until we had also found them confirmed by the examination of other volcanic regions. We found this confirmation above all in South America. External circumstances long prevented the publication of the results of our studies. Today, after more than 20 years, we have resolved to publish them, at least in part.

The island of Tenerife bears a certain resemblance to Madeira in its configuration, although it is more than twice the size in area. This resemblance arises chiefly from the fact that Tenerife, too, is made up of two parts which must be distinguished genetically, even though tectonically they are joined and interlocked.

On Tenerife, too, we see the island’s caldera-shaped elevation joined to an extensive, flattened elevation without a crater. However, these two parts do not stand in the same relation to each other as, on Madeira, the caldera of the Curral does to the dome of the Paul da Serra. On Tenerife the part containing a caldera is so extensive and so high that the rest appears merely as a lateral appendage, a secondary member, which is not the case on Madeira.’