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Scattered Mines .

_’ Zomelahuacan; Giliapa; San Antonio de
Xacala.

XII. Old Calijbrnia;
Mine. Real de Santa Ana.

Those who have studied the geological constitution of a mining country of great extent, know.the difliculty of reducing to general ideas the observations made on a great variety of beds, and metalliferous veins. The naturalist may distinguish the relative antiquity of the di-fiereut jbrmations, and he is enabled to discover laws, in the stratification of rocks, in the identity of beds, and often even in the angles which they form, either with the horizon or the meridian of the place; but how can he recognize the laws which have determined the disposition of the metals in the bosom of the earth, the extent, the direction, and inclination of the veins, the nature of their mass, and their particular structure? Howi canr he draw: general results from the observation of a multitude of small phenomena, modified by causes of a purely local nature, and appearing to be the effects of an action of chernical_. aflinities, confined to awvery narrow, space? ; -These difficulties are increased when it happens,]as ic mountains of Mexico, that the veins,

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the beds, and the masses, (slockwerke) are scat-
tered in an infinity of mixed rocks of very
different formations. If we possessed an accu-
rate description of the four or five thousand
veins actually wrought in New Spain, or which
have -been wrought within the two' last_'cen--

- turies, we should undoubtedly perceive in. the
materials and structure of these veins, analogies
indicative of a simultaneous origin; we should
find that these -vein materials (gang-ausfwillungen)

. “are partly. the same with those which are exhi-
bited in the veins of Saxony and Hungary,
and on which M. Werner, the first mineralo-
gist of the age, has thrown somuch light.“ But ‘
we are yet very far from being acquaintedrwith
the metalliferous mountains of Mexico; and not-
withstanding the great number of observations
collected. by myself in travelling through the
country in different directions, for ai length of
more than 4<00 leagues, I shall not venture to
sketch a. general view of the Mexican mines,
considered. under their geological relations, I
'sh“all~ 'c‘ontent myself with merely indicating

i the rocks 'which~yield the greatest part of the-
wealth of New'Spain. t ' V

In the present state of the country, th~
are the object of the most considerable opera-
tions; and the ores disposed 'in beds or in~
masses are not frequent. :1‘hegMexican veins (V VA
§L§..-EQIL..$.h...e.._11b_1_f_>_§j5_ papt found in ' "I I aiidv I

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130 POLITICAL ESSAY ON THE [noon iv.

I
transition rocks (ur-und iibergangs-gebiirge),
and rarely in the rocks of secondary formation,
which only occupy a vast extent of ground to
the north of the Tropic of Cancer, to the east
of the Rio del Norte, in the basin of the Missis-
sippi, and to the west of New Mexico, in the
plains watered by the rivers of Zaguananas
and San Buenaventura, which abound in
muriatic salts. In the old continent granite,
gneiss and micaceous slate (glimmer-schirjér)
constitute the crest of high chains of mountains.
But these rocks seldom basket out on the
ridge of the Cordilleras of America, particu-
larly in the central part contained between the
18°, and 22° of north latitude. Strata of am-
phibolic porphyry, greenstone, amygdaloid, ba-
salt and other trap formations of an enormous

thickness, cover the granite, and conceal it from

the geologist. The coast of Acapulco is formed
of granitic rocks. Ascending towards the table
land of Mexico,we see the granite piercethrough
the porphyry for the last time between Zum-
pango 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 auriferous veins.

Tin, which after Titanium, Scheelin, and Mo-
lybdena,is the oldest metal of the globe, has never
yet,as far as I know,becn observed in the granites
of Mexico ; for the fibrous tin (wood-tin) of the
\._.,‘\_V\__\\_ \ ‘ ‘Ag

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Gigante belongs to alluvial rocks, and the veins
of tin of the Sierra de Guanaxuato are found in
mountains of porphyry. In the mines of Co-
manja, asyenite, apparently of antient formation,
contains an argentiferous vein. That of Gua-
naxuato, the richest of all America, traverses a
primitive slate (tlzon-sckiefe‘r) which frequently
passes into tall:-slate (talk-schider). “Fhe
serpentine of Zimapan appears destitute of
metals. 1 +

. The porplzyries of Mexico may be considered
for the most part as rocks eminently rich in
ores of gold and silver. One of the problems
of geology, the most difiicult to resolve, is the
determination of their relative antiquity. They
are all characterised 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 feldspar 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 vitre-
ous fizldspar in the porphyries of Spanish
America. The rock which is intersected by
the rich gold vein of Villalpando, near Gua-
naxuato, is a porphyry, of which the basis is
somewhat akin to /tlingstein (phonolite), and

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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. de Born by the very vague denomination of sawum metalli/‘cram. The veins of Zimapan, which are the most instructive in respect to the theory of the repositories of minerals, traverse porphyries of a. greenstone base which appear to_belong to trap rocks of a newer formation. These veins of Zimapan offer to oryctognostic collections a great variety of interesting minerals, such as fibrous zeolite, stilbite, grammatite, pycnite, native sulphur, fluor spar, barytes, suberiform asbestus, green garnets, 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 fire opal. . V i Among the transition rocks which contain ores of silver, we may mention the transitionlimestone (iibergangs-lcallcstein) of the Real del Cardonal, of Xacala, and of Lomo del Toro, to the north of Zimapan. In the last, of these places what is worked are not veins, but masses of galena, of which some have yielded in a short space of time, according to the observation of M. Sonneschmidt, more than 124,000 quintals of lead. The grauwaclre alternating

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