Punta Concepcion, Mulegé Municipality, Baja California Sur, Mexicoi

Regional Level Types
Punta Concepcion	Peninsula
Mulegé Municipality	Municipality
Baja California Sur	State
Mexico	Country

In Lower California, on the western shore of the Gulf, between longitude 111 degs. and 112 degs. west and latitude 26 degs. and 27 degs. north, running due north-east and south-west, is a small peninsula about 25 miles in length, and varying from 2 to 6 miles in breadth, whose northern extremity lies a few miles east of the small town of Mulege, while it is joined to the mainland on the south. Mulege is well watered, and forms a delightful oasis of semi-tropical fruit and other trees in a veritable desert of volcanic rock. About 43 miles north of the town are the well-known Boleo copper mines and works, owned by a French company. The whole coast in this district is formed of eruptive rock, and has an exceedingly sombre and desolate aspect, looking pleasing only by moonlight, or when coloured by the rays of the setting sun. Mulege can be reached by a small steamer or sailing boat from the port of Guaymas, Sonora, on the eastern side of the Gulf of California.

The small peninsula appears to consist of trachytic rock, either in hard solid beds, as a soft tufa, or as a conglomerate. The only vegetation besides the cactuses and mesquite so characteristic of Northern Mexico, occurs in the ravines or quebrados, and consists mainly of small trees known as palo fierro (iron wood), palo bianco (white wood), depuas, matacoras (a shrub with yellow leaves and a blue flower, yielding a brownish-black dye), torotis (a similar shrub with white flowers, and yielding a red dye), jitos, etc. The fauna is if possible even more limited than the flora, being confined principally to a small species of deer, jack rabbits, vultures, and rattlesnakes.

In the geological sketch-map of Mexico, issued by the Government, eruptive rocks are marked only at Cape Virgenes to the north, and at and near La Paz in the south, showing that the labours of the Commission along the coast-line of Lower California were not of an exhaustive character. The fact is, the geology of Mexico has still to be worked out, the knowledge of the northern portion especially being extremely limited.

In the spring of last year the writer had an opportunity of examining the small peninsula of Mulege for manganese ore, and although the deposits proved to be of no great commercial value, it is believed that they are of some geological interest.

At a place called Guadolupe, on the foothills, 150 feet above sea-level, and some 1,200 yards inland, are various outcrops of manganese ore.

Several veins are seen here coursing through the trachytic country. One runs east 21 1/2 degs. south to west 21 1/2 degs. north,† and another north 40 degs. west to south 40 degs. east for some yards, it then runs north and south.

The veins consist of psilomelane and sulphate of lime (gypsum), running through rock much stained by oxide of iron. Another vein runs north 27 degs. east to south 27 degs. west, dipping almost perpendicularly—on its eastern side the redstained "country" has parallel veins of manganese running through it for a width of 4 1/2 feet—it then turns east 28 degs. north to west 28 degs. south, and further north is very poor, being much mixed with trachyte, sulphate of lime, and some iron.

The bearing of another vein was west 42 degs. north to east 42 degs. south, and its dip east 50 degs. The country consists of moderately hard red trachyte or porphyry, passing occasionally into harder white trachyte.

Near this spot, on a hill 250 feet above the sea, is an outcrop of small veins of manganese ore, separated by varying thicknesses of country rock. Some of the veins branch into, and some cross each other, and all appear to dip very flatly to the east. The deposit, in fact, is in the nature of a "stock-work." One vein consisted of a layer of psilomelane, 1/4 inch thick on each wall, with quartz from 1 to 4 inches thick, forming vughs in the centre.

One vein runs east 30 degs. north to west 30 degs. south, dips north 64 degs., and is 2 1/2 feet thick at its outcrop, and has a more or less solid appearance. A close examination, however, shows sulphate of lime running through it in veinules, and also in crystals or crystalline patches. Another vein, close to it, running east 10 degs. south to west 10 degs. north, and dipping south 57 degs., was 3 1/2 to 4 feet wide.

In the same locality a vein had the section shown in Fig. 3, Plate LX, consisting of layers of crystalline manganese ore, gypsum, and quartz.

To sum up, the veins here vary in thickness from a few inches to 3 or 4 feet, and course from north 27 degs. east to east 42 degs. south, although the prevalent direction would appear to be north-west nearly. The contents are manganese peroxide, varying from almost pure to a mixture of manganese ore with much foreign matter, red trachyte in layers and patches, sulphate of lime in layers and patches, oxide of iron, and quartz. In the hard trachyte the veins are very poor, and show little or no quartz. The most prevalent gangue or foreign matter is sulphate of lime, in fact it is not easy to obtain a specimen of ore that does not contain some.

At Aguja Point, at the north end of the peninsula, a bed of trachytic conglomerate 20 feet thick is seen lying on red trachyte. The rocks here trend north and south, and dip east 22 degs.

On the top of the cliffs at Los Pilaris, or about 200 feet above the sea, an outcrop of hard red trachyte is seen traversed for about 100 feet in width by veins of hard crystalline manganese ore with sulphate of lime, quartz, and calcite, from mere threads to a few inches thick, and separated by much rock. In the "canada" or glen below this outcrop some large lumps (boleos) of almost solid ore have been found lying in the loose ground. These are undoubtedly the result of surface denudation, and have no doubt come from the points of contact of two or more veins where swellings may be expected to occur.

The chief veins here run north 35 degs. west to south 35 degs. east, and dip west at a high angle (in one case west 70 degs.), but some dip east. One vein was from 3 to 3 1/2-inches thick, but it thinned off to the thickness of a knife blade a few feet on either side. Three and a half feet from this was another vein 1 inch thick. Some of the veins for a foot or two in length are filled with oxide of iron.

The chief veins, or more probably joints, course in wavy lines, consisting of short curves of a few feet in circumference only. The cross veins are very thin and also run in wavy lines. Occasionally junctions occur between the veins. The ore here is much purer than at Guadolupe, being generally free from sulphate of lime, but the veins are very thin, seeming to form a kind of stock-work in hard rock.

At Gavelan, green trachytic tufa forms the country. This rock is probably simply the result of surface decomposition of trachyte, and here the veins or main joints run north-west and dip south-west. The rock is soft mottled yellow and green tufa, with soft and crystalline manganese
(pyrolusite), with some pink mineral (? rhodonite or rhodochrosite). At the point known as Punta Amarilla, thin beds of yellow volcanic rock form the cliffs, and appear to run east and west, and to dip north 45 degs. (readings taken from the ship).

At La Trinidad, the best and most solid ore has been found. Here 500 yards inland and about 100 feet above sea-level two almost north and south veins intersect, and each vein near the point of intersection is ore-bearing. The country is more or less soft red, white, and yellow rock, almost like clay, and much stained by oxide of iron. It appears to be in reality decomposed trachyte.

The lowermost vein runs north 14 degs. west, turns about 13 degs., and then crosses the chief one which runs north 8 1/2 degs. east, both dip westerly. At the junction of the two veins there is a "formacion" of soft red rock and manganese about 12 feet thick. On the hanging wall side there are about 1 1/2 feet of good ore, soft and friable (pyrolusite), but the ore towards the centre of the vein and on the footwall side is much mixed with iron.

The ore appears to come in in lenticular masses, whose axes lie parallel to the dip of the vein, and which close rapidly both vertically and horizontally.

The lower vein also contains some good ore near its junction with the upper one, but some little distance below the 'point of intersection both veins contain much iron and are very impure.

At Santa Rosa, red trachytic tufa or felspathic ash is seen striking north and south and dipping west 57 degs. Here the main joints run east and west to east-northeast and west-north-west, and dip south. Not unfrequently they contain manganese as a mere discoloration, and oxide of iron, or the former mineral well crystalline and a few inches thick. Cross joints, dipping east 80 degs., are also lined with manganese. In places, both sets of joints are well pronounced, splitting the rock up into pencil-like fragments. Some of the joints are filled with light-coloured ash.

At Santa Teresa, manganese ore is seen coating the joints of the rock in very thin films. The rock here is very hard trachyte, and one specimen of ore picked up showed sulphate of lime, resembling the Gruadolupe mineral. In another part of the peninsula, where the trachyte was white or grey, weathering to red, the rocks appeared to strike east-north-east, and to dip west 50 degs.

In a canada near Los Pilaris, grey trachyte is seen below the red, then come 2 feet of trachytic conglomerate, followed by several yards of red porphyritic conglomerate. The latter contains oxide of iron, and little veins and patches of manganese. Here and there joints containing clay open out into pockets of blue and green carbonate of copper, sulphate of lime, and talc. The main joints or fissures in this instance run north 20 degs. east and dipped east 60 degs.

The interior of the peninsula was not examined, but it appeared to be formed of a belt of hard eruptive rocks, and the chance of finding manganese in payable quantities therein was considered remote.

No distinct evidence of any true fissure-vein was seen. The manganese ore appears to occur in stock-works, and filling superficial vein-like fissures and rock joints. The manganese ore probably comes from the trachyte itself, and is the result of surface erosion and decomposition. The ore has probably been leached out of the rock, and has subsequently been deposited in the surface fissures and joints.

The veins at Guadolupe which contain sulphate of lime, possibly owe their origin to hot springs. About 60 miles north there is an extinct volcano (el Volcande las Virgenes), where hot springs are now depositing sulphur and gypsum.