GEOLOGICAL STRUCTURE.

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ling, and that from 1786 to 1803, the annual average produce has been 342,231 lbs. troy of silver, equal to 1,059,6551. sterling. All the veins of Hungary and Transylvania together, yield only 55,686 lbs. troy of silver.

Taking four averages of years, of which three are of ten and one of eight years, we shall have the following results :

The rocks known in the district of Guanaxuato, are clay-slate and chlorite-slate (killas), which are considered to be of primitive formation.

In sinking the great shaft at the Valenciana mine, they went through floors of syenite, hornblendeslate, and true serpentine.

There are resting on some parts of the clay-slate of this district, porphyry and old sandstone. The porphyry forms gigantic masses on the surface. The sandstone occurs at lower elevations, and is a breccia with an argillaceous cement.

This formation serves again as a basis to other secondary beds, such as limestone and gypsum. Floors or courses of greenstone are also sometimes found in the primitive rocks.

Such is the geological constitution of the country at Guanaxuato. The vein (veta madre) traverses clay-slate as well as porphyry; and in both of these rocks it has proved very rich in ore. Its mean direction is h. 8 of the miner's compass*; and is nearly the same with that of the veta grande of Zacatecas, and of the veins of Tasco and Moran, which are all western veins (spathgänge). The inclination of the vein of Guanaxuato is 45 or 48 degrees to the south-west. We have already stated, that it has been wrought for a length of more than 39,000 feet; and yet the enormous mass of silver which it has supplied for the last hundred years,sufficient of itself to produce a change in the price of commodities in Europe,-has been extracted from that part of the vein alone contained between the shafts of Esperanza and Santa Anita, an extent of less than 8500 feet. In this part we find the mines of Valenciana, Tepayac, Cata, San Lorenzo, Animas, Mellado, Fraustros, Rayas, and Santa Anita, which at different periods have enjoyed great celebrity.

The veta madre of Guanaxuato exhibits the extraordinary example of a fissure formed accord

* Or N. 52°. W.

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ing to the direction and inclination of the strata of the rock. Towards the south-east from the ravine of Serena, or from the mines of Belgrado and San Bruno, which are very fully wrought, to beyond the mines of Marisanchez, it runs through porphyritic mountains; and towards the north-east, from the shafts of Guanaxuato to the Cerro de Buena Vista and the Cañada de la Virgen, it traverses clay-slate (thon schiefer). Its magnitude varies like that of all the veins of Europe. When not ramified, it is generally from 40 to 50 feet in breadth; sometimes it is even contracted to the width of 20 inches; and it is for the most part found divided into three masses (cuerpos), separated either by rock (caballos), or by parts of the gangue almost destitute of metals. In the mine of Valenciana the veta madre has been found without ramification, and of the breadth of 22 feet, from the surface of the ground to the depth of 93 fathoms. At this point it divides into three branches, and its extent, reckoning from wall to wall of the entire mass, is from 160 to 200 feet. Of these three branches of vein, there is in general but one which is rich in metals; and sometimes when all the three join, as at Valenciana near the pit of San Antonio, at a depth of 164 fathoms the vein contains immense riches to a width of more than 80 feet. In the pertinencia de Santa Leocadia, four branches are observable. A branch, of which the inclination is 65°, separates from the

inferior branch (cuerpo baxo), and cuts the rock of the wall.

The small ravines into which the valley of Marfil is divided, appear to have a decided influence on the richness of the veta madre of Guanaxuato, which has yielded the most metal where the direction of ravines and the slope of the mountains Alaqueza del cerro) have been parallel to the direction and inclination of the vein. When we stand on the elevation of Mellado, near the shaft which was sunk in 1558, we observe that the veta madre is in general most abundant in ores towards the north-west, towards the mines of Cata and Valenciana; and that to the south-east towards Rayas and Santa Anita, the produce has been at once richer, rarer, and more inconstant. Besides, in this celebrated vein there is a certain middle region which

a

may be considered as a depositary of great riches; for above and below this region, the ores have yielded an inconsiderable share of silver. At Valenciana the rich ores have been in the greatest abundance, between 55 and 186 fathoms in depth below the mouth of the gallery. This abundance appeared at Rayas at the surface of the earth; but the level of Valenciana is pierced, according to my measurements, in a plain which is more than 511 feet above the level (galerie d'ecoulement) of Rayas; which might lead us to believe that the depositary of the great wealth of Guanaxuato is found in this part of the vein, between 6987 and 6199

CONSTITUENT PARTS OF THE VEIN. 239

feet of absolute height above the level of the ocean. The deepest works of the mine of Rayas (los planes) have never yet reached the lowest limit of this middle region; while the bottom (das tiefste) of the mine of Valenciana, the level of San Bernardo, has unfortunately passed this limit more than 230 feet. Hence the mine of Rayas continues to furnish extremely rich ores, while at Valenciana they have endeavoured for some years to supply by the extraction of a greater quantity of ores the deficiency in their intrinsic value.

The mineral substances which constitute the mass of the vein of Guanaxuato, are common quartz, amethyst, carbonate of lime, pearl spar, splintery hornstone, sulphuret of silver, dendritic native silver, prismatic black silver, deep red silver, native gold, argentiferous galena, brown blende, spar iron, and copper and iron pyrites. We observe besides, though much more rarely, crystallized feldspar (the rhomboidal quartz of the Mexican mineralogists), calcedony, small masses of sparfluor, capillary quartz (haarformiger quartz), gray copper ore (fahlerz), and columnar carbonate of lead. The absence of the sulphate of barytes and muriate of silver distinguishes the formation of the vein of Guanaxuato from that of Sombrerete, Catorce, Fresnillo, and Zacatecas. Those mineralogists who are interested in the study of regular forms, find a great variety of crystals in the mines of Guanaxuato, and especially in the red and black