PETEOGRAPHIOAL CHARACTER OF NEGATTNEE FORMATION. 369 



quartz developed. Tlie griuierite and inagnetite are closely asscxnated, 

 often penetrating' each other, and are also found within and penetrating the 

 quartz, showing that the minerals developed to some extent simultaneously, 

 although the quartz appears on the whole to be somewhat later than the 

 griinerite and magnetite. In the finer-grained phases opaline silica is also 

 present. As in the case of the sideritic slates, some of the griineritic 

 schists contain interstratified or intermingled fragmental material, and the 

 rock by transition passes downward into the fragmental Siamo slate In 

 these kinds ordinary hornblende has abundantly developed, and chlorite 

 and biotite are important secondary products. Sometimes associated or 

 ineludeil in the magnetite-griinerite-schists is a great deal of secondary 

 garnet, and this is particularly abundant adjacent to greenstones, slio^ving 

 that its development is related to the intrusives (see pp. 513-514). 



The development of the griiuerite-magnetite-schists, in contrast with 

 the ferruginous slates, cherts, and jaspers, seems to have been favored by 

 deep-seated metasomatic changes, rather than by Aveathering processes. 

 This is indicated by the following tacts: A¥here weathering has been active, 

 the ferruginous slates and cherts are found rather than the griinerite-mag- 

 netite-schists; the griinerite-magnetite-schists where weathered have been 

 partly transformed into the ferruginous slates or cherts; the griinerite- 

 magnetite-schists are usually closely associated with the greenstones. This 

 suggests that the heat of these intrusives increased the activity of i)erc-olat- 

 ing waters; possibly also the heat helped to decompose the iron carbonate-, 

 and the greenstones may also have furnished alkalis to assist in the solu- 

 tion of sihca. The silica in solution united with the protoxides present to 

 produce the griinerite and other amphiboles, the excess of iron oxide, not 

 completely oxidized, remaining as magnetite. 



The ferruginous slates (PI. XIX) consist of cherty silica, like that of 

 the sideritic slates, and of hematite and limonite, the latter minerals occu- 

 pying the place of the siderite in the sideritic slates. AVhere the iron 

 oxides are abundant the slides consist of a continuous, ramifying mass of 

 hematite and limonite, Avithin which the numerous ])atclies or particles 

 of cherty silica are set. Where the silica is aliundant the reverse relations 

 MON XXVIII 24 



