ALMADEX. 29 



discovered from time to time, but are said now to be exhausted or abandoned 

 for other reasons. 



The prevailing rocks of the i\.hnaden district are scliists, quartzites, 

 and sandstones, together with small quantities of limestone, all of Silurian 

 and Devonian age. Intimately associated with the deposits, though seldom 

 in direct contact with the ores, is a rock called piedra frailesca. According 

 to de Prado this is a metamorphosed breccia, consisting of grains of quartz, 

 calcium carbonate, dolomite, and fragments of schist cemented by dolomitic 

 calcite. It occurs in lenticular masses intercalated in the schists and has 

 been found to contain Silurian fossils. Messrs. Helmhacker and Calderon 

 regard the rock as a diabase tufa. Cracks in this rock sometimes carry 

 cinnabar, the deposition of Avhich is therefore later than the brecciation. 



The district lies upon the northern flank of the Sierra Morena. In 

 this range are extensive areas of granite, and a rock also called granite 

 crops out at various points not many miles to the north of the mines. Di- 

 abase, or meiaphyre, has broken through the sedimentary rocks and occu- 

 pies considerable areas near the mine, and a small quantity of porphyrv, 

 regarded as trachy tic by de Prado, but as Pre-Tertiary by more recent Span- 

 ish geologists, exists some six miles northeast of Almaden. The sediment- 

 ary rocks are nearly vertical and are said to be little disturbed by the 

 diabase eruptions, which have naturally readied the surface along the 

 planes of bedding. The strata carry enough fossils for a satisfactory de- 

 termination of the age of the rocks as a whole, but the same beds seem to 

 reappear more than once in the compressed folds, and it is often difficult to 

 decide to which of the periods a particular stratum belongs. 



The Almaden district contains many deposits of cinnabar scattered 

 over an area of about ten miles by six, but neither these nor the ranges of 

 hills exactly follow the strike of the strata, which is very closely east and 

 west. There seems to be some tendency, however, both with the deposits 

 and the ranges, to arrangement in the same direction. 



The chief ore is of course cinnabar, accompanied by relatively small 

 quantities of metallic mercury. Pyrite occurs in small quantities, and 

 Caron detected chalcopyrite. Gangue minerals have been said to be al- 

 most entirely wanting, but Noggerath detected a little lieavy spar witli the 



