Geology and Natural History. 409 



origin than the porphyries. The mineral veins lie in distinct 

 areas or belts. The greatest contrast in them is offered by those 

 which are chiefly silver-bearing as compared with those which 

 are predominantly gold-bearing. 



As a result of the investigation of the gold, silver and lead 

 deposits of Colorado two chief natural groups appear, those 

 following injections of a monzonite magma including a large part 

 of the mineral zone with northeast trend across the state, and 

 those which followed isolated local outbursts of magma of an 

 alkalic character. Both periods of ore deposit are of Tertiary 

 age. The ores of the Georgetown district belong in the former 

 northeast belt of which they are a portion. The gold ores are 

 pyritic ; the silver ores are galena-blende. Areas covered by 

 the veins coincide with those injected by porphyry dikes and the 

 ore formation is believed to have immediately followed the 

 intrusions. The gold ores depend definitely upon magmas pro- 

 ducing one family of igneous rocks, the silver ores upon a dis- 

 tinctly different magmatic family. 



Further study of the deposits leads to the belief that the ores 

 have been formed in the case of the pyritiferous gold veins by 

 magmatic waters given off during the pneumatolytic stages of 

 consolidation of certain dikes, due to the intrusion of alkaline 

 magma, while the deposition of the silver-lead ores has followed 

 in a similar way from monzonitic magmas. Since the latter 

 deposits, however, are the older they have suffered much erosion, 

 and in their present status represent the action of a secondary 

 period of concentration and enrichment from descending surface 

 waters. 



In connection with this the waters of certain hot-springs have 

 been studied and, from their composition and the mineral reac- 

 tions which they produce, it is concluded that they are of mag- 

 matic origin. Many of the minerals which they form are due to 

 reactions with the wall rocks with which they come in contact 

 and thus wall rocks of differing composition have produced dif- 

 ferent sets of minerals. The general conclusion is drawn from 

 the study that the greater portion of the gold, silver and lead 

 ores of Colorado are of magmatic origin and this mode of ore 

 formation, that is their being due to emanations from igneous 

 magmas, is one of first importance, in comparison with which all 

 other modes of formation, save in the case of the most common 

 metals, shrinks into insignificance. l. v. p. 



3. Geology of the Gold Fields of British Guiana; by J. B. 

 Haerisox. 8°," pp. 320, 33 pis. London, 1908 (Dulau & Co.).— 

 While this volume is ostensibly devoted to a description of the 

 occurrences of gold ores in the colony, it is for the most part made 

 up of a report on the results of reconnaissance work following 

 explorations along several important rivers ; it contains in addi- 

 tion a considerable amount of careful, thorough and excellent 

 petrographical research made upon the rock types collected. 

 Especially to be commended are the very complete and accurate 

 analyses which have been carried out on a considerable number 



