654 REVIEWS 



either of low grade or of small extent. Only two really important depos- 

 its are worked. 



The important manganese deposits of Costa Rica are in the Nicoya 

 Peninsula, which is very hilly, with a "backbone" running in a generally 

 northwesterly direction. The east coast (Nicoya Gulf) is low, with 

 swamps and estuaries, while the Pacific coast is high and rugged. The 

 rocks are chiefly sandstone, shale, conglomerate, and limestone. Most 

 of the sediments have undergone considerable dynamo-metamorphism, 

 the greater part being now iron-pi gmented quartzite, but other less 

 highly colored quartzite occurs higher in the sequence. Silicified Hme- 

 stone, shale, and breccia are also reported. Igneous rocks include basic 

 fine-grained types, largely flows (?). Some intrusions are also thought 

 to characterize the region though no plutonic rocks are actually de- 

 scribed. Structurally the area seems to indicate an igneous basement, 

 with superjacent sediments that have been intricately folded and faulted 

 since deposition. 



The ore-bodies are manganese oxides, partly soft (pyrolusite?), 

 partly hard and crystalline. Iron oxide is generally low, but silica occurs 

 mechanically admixed. The oxides are found in pockets or troughs 

 between the red metamorphosed rocks and lighter colored sediments, or 

 may be in direct contact with igneous rock. Generally the deposits are 

 too small to merit another term than "pocket," but they may be as 

 large as 500 by 100 feet, averaging 5 feet in thickness. The exact size 

 of these ore-bodies is not determinable, and estimates of a reserve based 

 on a 40-45 per cent ore are therefore not dependable. 



The ores are related to fault zones but not all the faults of the region 

 are ore-bearing. The ore is attributed by the writer to hydrothermal 

 action, the hot, ascending solutions passing along the faults and spreading 

 on planes of contact between formations and depositing the manganese 

 as a carbonate or silicate which was later oxidized. The great silicifica- 

 tion of the wall rock and close relation between the ore and fissures are 

 supposed to lend credence to this view. On the other hand the manga- 

 nese may be the product of downward concentration, deposited because 

 of the impermeabihty of the highly metamorphosed rocks. 



A detailed discussion of the mines and prospects emphasizes their 

 economic insignificance. Two only are of importance as producers at 

 present, those at Playa Real and at Curiol, and one prospect (Pavones) 

 may prove to be productive in the future. 



In Panama two manganese deposits are known on the west side of 

 the Boqueron River, about 20 miles east of Colon. The country near 



