The red notebook of Charles Darwin / edited, with an introduction and notes by Sandra Herbert.

  • Charles Darwin
Date:
1980
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: The red notebook of Charles Darwin / edited, with an introduction and notes by Sandra Herbert. Source: Wellcome Collection.

    142/184 (page 128)
    128 SANDRA HERBERT Other trap formations of an enormous thickness cover the granite and conceal it from the geologist. The coast of Acapulco is formed of granite rock. Ascending towards the table land of Mexico we see the granite pierce through the porphyry for the last time between Zumpango and Sopilote. Farther to the east in the province of Oaxaca the granite and gneiss are visible in table lands of considerable extent traversed by veins of gold. Humboldt, Political Essay on the Kingdom of New Spain, vol. 3, p. 131. The original sentence reads The porphyries. . . Humboldt, Political Essay on the Kingdom of New Spain, vol. 3, pp. 131-132; They [the Mexican porphyries] are all characterized by the constant presence of amphibole and the absence of quartz, so common in the primitive porphyries of Europe, and especially in those which form beds in gneiss. The common felspar is rarely to be seen in the Mexican porphyries; and it belongs only to the most antient formations, those of Pachuca, Real del Monte and Moran, where the veins furnish twice as much silver as all Saxony. We frequently discover only vitreous felspar in the porphyries of Spanish America. The rock which is intersected by the rich gold vein of Villalpando near Guanaxuato is a porphyry of which the basis is somewhat a kin to klingstein {phonolite), and in which amphibole is extremely rare. Several of these parts of New Spain bear a great analogy to the problematical rocks of Hungary, designated by M. Born by the very vague denomination of saxum metalliferum. The veins of Zimapan which are the most instructive in respect to the theory of the stratification of minerals are intersected by porphyries of a greenstone base which appear to belong to trap rocks of new formation. These veins of Zimapan offer to oryctognostic collections a great variety of interesting minerals such as the fibrous zeolith, the stilbite, the grammalite, the pyenite, native sulphur, spar fluor, baryte suberiform asbestos, green grenats, carbonate and Chromate of lead, orpiment, Chrysoprase, and a new species of opal of the rarest beauty, which I made known in Europe, and which M. M. Karsten and Klaproth have described under the name of (feuer-opal). Humboldt, Political Essay on the Kingdom of New Spain, vol. 3, pp. 133-134: In proportion as the north of Mexico shall be examined by intelligent geologists, it will be perceived that the metallick wealth of Mexico does not exclusively belong to primitive earths and mountains of transition, but extend also to those of secondary formation. I know not whether the lead which is procured in the eastern parts of the intendancy of San Luis Potosi is found in veins or beds, but it appears certain, that the veins of silver of the real de Catorce, as well as those of the Doctor and Xaschi near Zimapan, traverse the alpine lime-stone (alpenkalkstein); and this rock reposes on a poudingue with silicious cement which may be considered as the most antient of secondary formations. The alpine lime-stone and the jura lime-stone (Jurakalkstein) contain the celebrated silver mines of Tasco and Teuilotepec in the intendancy of Mexico; and it is in these calcareous rocks that the numerous veins which in this