Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/446

336 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 9, 3. On the Caratal Gold-field.

By C. Le Neve Foster, B.A., D.Sc, F.G.S.

[Abridged.]

The Caratal goldfield, for our knowledge of which we are indebted to Dr. Plassard, is situated in Venezuela, about 160 miles in a straight line east-south-east of Ciudad Bolivar or Angostura, the capital of the State of Guayana. It lies about 100 miles south of the River Orinoco, about seventy-five miles up from the principal mouth.

Before entering upon a description of the various kinds of gold- deposits, I must give a short general sketch of the geology of the district.

The country between Ciudad Bolivar and Pastora consists almost entirely of gneiss, with some mica-schist and hornblende-schist, and a little granite. All three varieties of rock are seen at Ciudad Bolivar itself. From Ciudad Bolivar to the Caroni there is a gently undulating savannah showing outcrops of gneiss as far as the neighbourhood of the Arasiama ridge, which is formed of itaberite. About Arasiama, and on both sides of the Caroni near Guri, the strike of the gneiss is, roughly speaking, east and west.

Beyond Charapo there is some hornblende-schist striking E. 15° N. in some of the land which forms the watershed between the basin of the Orinoco and that of the Essequibo. Where the road crosses it, the watershed is not more than about 1100 feet above the sea-level.

Further east, at Limones, I saw granite; but the greater part of the country consists of gneiss, which now gets a strike of N. 18° W. to N. 15° E. Both east and west of the Oronato I found horn-blendic schist, striking N. 10° E., N. 45° W.

In crossing the Guatapolo I noticed a dyke of porphyrite resembling some of the Cornish elvans, and remarkable for the well-crystallized quartz which it contains.

Before arriving at Pastora the series of schists and slaty rocks, which continue into the Caratal district, make their appearance; near Pastora there are siliceous schists striking N. 45° "W., N. 60° W., N. 70° W.; and at Caratal itself these rocks are often a fine-grained clay-slate like some of the Cornish "killas." At other times the rock is much coarser; and a very talcose variety frequently occurs. As a rule, these slaty rocks are much decomposed. The decomposed schist forms the "bed-rock" in many of the alluvial diggings, and is known to the miners as "cascajo." Besides these slaty rocks, there are large outcrops of a felstone; but I cannot say whether it is intrusive or interbedded. This felstone is the "blue-stone" of Dr. Stevens*. It is a compact grey, bluish-grey, or greenish-grey rock, often containing small crystals of iron pyrites, the "mica" of the miners. The schistose rocks have, roughly speaking, an east and west strike, with a varying dip.

1st Aug. 1868.
 * 'Scientific American,' 25th Nov. 1868, and 'American Journal of Mining,'