[footnote] *Dufrenoy, in the 'Memoires Geologiques', t. ii., p. 145 and 179.
There is much difficulty in explaining the origin of the beds of pure
quartz, which occur in such large quantities in South America, and impart so
peculiar a character to the chain of
p 266
the Andes.*
[footnote] *Humboldt, 'Essai Geogn. sur le Gisement des Roches', p. 93;
'Asie Centrale', t. iii., p. 532.
In descending toward the South Sea, from Caxamarca toward Guangamarca, I
have observed vast masses of quartz, from 7000 to 8000 feet in height,
superposed sometimes on porphyry devoid of quartz, and sometimes on diorite.
Can these beds have been transformed from sandstone, as Elie de Beaumont
conjectures in the case of the quartz strata on the Col de la Poissonniere,
east of Brian??on?*
[footnote] *Elie de Beaumont, in the 'Annales des Sciences Naturelles', t.
xv., p. 362; Murchison, 'Silurian System', p. 286.
In the Brazils, in the diamond district of Minas Geraes and St. Paul, which
has recently been so accurately investigated by Clausen, Plutonic action has
developed in dioritic veins sometimes ordinary mica, and sometimes specular
iron in quartzose itacolumite. The diamonds of Grammagoa are imbedded in
strata of solid silica, and are occasionally enveloped in laminae of mica,
like the garnets found in mica slate.
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