iron. 623 



outcrop, even if samples be taken at frequent intervals. The analyses given 

 in Table III of this class are very inferior types, but some of them might 

 readily be benefited by jigging or washing, although these are not really 

 average lots. 



3. SEGREGATED CONCRETIONARY ORES. 



Given a porous sandy rock, in many places made up in large part of the 

 sandy iron ores just described, in other portions having a very direct connec- 

 tion with subjacent bands of the hard belt ores, and it is reasonable to expect 

 a noticeable amount of infiltration from below. This is the case in marked 

 degree in the tracts defined upon the Economic Map, and perhaps in other 

 similar exposures which have not been plotted there. The richness of the 

 resulting ores is probably very closely related to the nearness of the deposit 

 to a buried band of magnetite, other things equal; but there are other con- 

 ditions which may unfavorably affect the quality of the product to such an 

 extent as to possibly overcome the advantage of proximity to the original 

 source of supply. On the other hand, conditions especially favorable to seg- 

 regation may have induced excessive deposition at points remote from the 

 advantageous contact in particular cases. Besides, the segregated ores will 

 have their best expression in districts where the Fernandian rocks are not 

 exposed by the removal of the Cambrian strata in which these supplies occur. 

 For these reasons the richest accumulations must serve rather as indicators of 

 the presence of buried magnetites, instead of being themselves discovered by 

 means of the magnetite bands. At the same time the knowledge that a 

 given magnetite band passes beneath the Cambrian sandstones at any point 

 may be taken as presumptive evidence of the good quality of the local seg- 

 regations until the contrary has been proven. 



Appearances, or even crude estimates of weight, made by holding speci- 

 mens in the hand, are commonly deceptive, and only complete chemical an- 

 alyses of average pulp from large samples can be safely relied upon in form- 

 ing judgment of commercial values. The patches which are noted upon the 

 Economic Map are not accurately outlined, but they show many of the prom- 

 inent areas in which this class of ore is abundant. A number of analyses are 

 given in Table III from different tracts, to exhibit the great variations in 

 quality. Some of the material which runs very low in iron can not be dis- 

 tinguished without much experience from the samples which are rich in that 

 metal. The rock does not always appear like an iron ore, and in some regions 

 it is covered with a glossy black coating of manganese oxide. It differs 

 from the sandy ores in being non-granular, usually very tough and compact, 

 and often having delicate impressions of shells of Lingula and allied forms of 

 Cambrian fossils. 



