552 REPORT— 1882. 



3, Summary of Rej)orts of the Committee on Undergroimd Temperature. By 

 Professor J. D. Everett, F.B.S. — See Reports, p. 74. 



4. Notes on the Geology and Mining of the United States of Colombia, S. A. 



By Robert B. White. 



Tliese notes refer specially to the States of Cauca and Antioqnia, whieli 

 comprise the country between the 1st and bth degrees of north latitude, and extend 

 from 120 to 150 miles eastward from the Pacific coast. The eastern and western 

 chains of the Andes, which run from south to north through this region, are distinct 

 hi their general characters. The eastern chain is almost all volcanic, whilst in tlie 

 western there are no volcanoes north of the 2ud degree, and the rest of the Cordil- 

 lera is composed of granites, granitoid rocks, and diorites. The upheaval of the 

 eastern chain has raised the strata of the Cretaceous formation to a height of 

 8,000 feet above the sea-level ; but on the western slopes of the western range 

 these strata are found only a few hundred feet above the sea, and are almost 

 undisturbed. 



Eruptions of igneous rocks have taken place in all ages between the two chains 

 of mountains, and the rocks of the valley of the river Cauca, which with its tributaries 

 occupies the space enclosed by the two Cordilleras, are metamorphosed and crystal- 

 lised to an extraordinary degree. 



The formation of an immense number of metalliferous veins seems to be the 

 natural result of this development of igneous rocks. 



The sedimentary strata are Laurentian, Silurian, Jurassic, Cretaceous, Tertiary, 

 and Post-tertiary. The Cretaceous beds contain good coal, limestone, and iron 

 ores. The Post-tertiary alluviums are nearly all auriferous, but although the 

 upper beds have been worked, the bed rock or old river channels have not yet been 

 sought after. It is probable that they are as rich as those of California. In recent 

 alluviums in the valley of the Cauca, remains of Mastodons have been found under 

 circumstances which indicate that tliese animals were coexistent with man. Pla- 

 tinum, iridium, molybdenum, copper, lead, and zinc are found in workable quan- 

 tities. G-old and silver are very abundant, and are the only metals mined for. It 

 is not a fact that platinum has ever been found in a lode in Colombia, but the author 

 has recently found iridium in appreciable quantity in a gold lode in the State of 

 Antioqnia. Diamonds are known to exist. liubies and sapphires of large size are 

 to be found in abundance in the State of Cauca, but the natives have not tried to 

 turn this wealth to account. 



_ The gold-mining is very interesting. Besides being found in the alluviums, gold 

 exists in lodes of all ages, from the oldest granites up to a post-cretaceous period. 

 The formation of gold and silver lodes in most abundance took place after the cre- 

 taceous period. In other countries the gold veins are usually confined to a limited 

 group of rocks. The Silurian is usually considered to be the principal formation 

 for gold, but in Columbia it is quite of secondary importance, although no doubt a 

 great part of the alluvial gold was derived from the denudation of the older rocks 

 which was effected upon the grandest scale imaginable. There is a great difference 

 in the standard or fineness of the gold, according to its age, and the oldest gold is 

 the best. It is found ranging from twelve to twenty-three and a half carats fine. 

 According to the age of the lodes diflerent metals are predominant in their associa- 

 tion with the gold. In the oldest lodes copper is most common, and in the newest 

 lead is the principal companion of the precious metals. When pyrites and galena 

 are plentiful in a well-constituted auriferous lode, it is generally found that the gold 

 will rather increase than diminish in quantity as the workings deepen. But lodes 

 which are not well-mineralised are most often richest on the surface. Common 

 arsenical pyrites is not a good companion for gold when it is not accompanied by 

 other sidphides. Pyrites is a general companion of gold, but it is not every class 

 of pyrites that is so, and lodes of different ages are characterised by different classes 

 of pyrites. There are also several varieties of galena, which are more or less 



