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.

    146/184 (page 132)
    132 SANDRA HERBERT position of the rock which crosses the veins of Guanaxuato? These questions are of so great importance that I must here give a geological view of so remarkable a country. The most ancient rock known in the district of Guanaxuato, is the clay slate {thon schiefer).... It is of an ash-grey or greyish-black frequently intersected by an infinity of small quartz veins, which frequently pass into talk-state [sic] {talk schiefer) and into schistous chlorite. Also, pp. 177-178: On digging the great pit {tiro general) of Valenciana, they discovered banks of syenite of Hornhlend slate {Hornblend schiefer) and true serpentine, altering with one another, and forming subordinate beds, in the clay slate.. . . These strata [of clay slate] are very regularly directed h. 8 to 9 of the miner's compass; they are inclined from 45 to 50 degrees to the south west.. . .Two very different formations repose on the clay slate: the one of porphyry. . .and the other, of o\á freestone in the ravins, and table lands of small elevation. And pp. 179- 180 : This porphyry... is generally of a greenish colour.... The most recent [beds],.. contain vitreous felspar, inchased in a mass, which sometimes passes into the petrosilex jadien, and sometimes into the pholonite [sic] or klingstein of Werner.. . , All the porphyries of the district of Guanaxuato possess this in common, that amphibole is almost as rare in them as quartz and mica. Humboldt, Political Essay on the Kingdom of New Spain, vol. 3, pp. 180-183: This free-stone {urfelsconglomerat) is a brescia with clayey cement, mixed with oxide of iron, in which are imbedded angulous fragments of quartz, Lydian stone, syenite, porphyry, and splintery hornstone.. . .This formation of old free-stone is the same with that which appears at the surface in the plains of the river Amazon, in South America.. . . We must not confound the brescia which contains imbedded fragments of primitive and transition rock, with another freestone, which may be designated by the name of felspar agglomeration.. . . This agglomeration... is composed of grains of quartz, small fragments of slate, and felspar chrystals, partly broken, and partly remaining untouched.. . .Probably the destruction of porphyries has had the greatest influence on the formation of thìs felspar freestone. It contrasts with the freestone of the Old Continent, in which some chrystals of grenats and amphibole have been found, but never. . .felspar in any abundance. The most experienced mineralogist, after examining the position of the lozero [agglomeration] of Guanaxuato, would be tempted to take it at first view, for a porphyry with clayey base, or for a porphyritic brescia {trümmer-porphyr).. . .These formations of o\d freestone of Guanaxuato, serve as bases to other secondary beds, which in their position, that is to say in the order of their superposition, exhibit the greatest analogy with the secondary rocks of central Europe. In the plains of Temascatio. . .there is a compact limestone. . Humboldt, Political Essay on the Kingdom of New Spain, vol. 3, p. 185 : The vein {veta madre) [of Guanaxuato] traverses both clay slate and porphyry. In both of these rocks, very considerable wealth has been found. Its mean direction is. . .[N. 52° W.] and is nearly the same with that of the veta grande of Zacatecas, and of the veins