416 THE PEJSIOKEE IRON-BEAEING SEKIES. 



in several detached areas. They are all closely associated with greeiistone- 

 couglomerates. That in the northeast corner of Sec. 24 is apj^arently in the 

 midst of a greenstone-conglomerate area; that in the north part of 8ec. 15 is 

 about lialf way between two known greenstone-conglomerate exposures; while 

 the exposures in the west part of Sec. 13, and in Sec. 14, are bounded upon tiu^ 

 south and west by greenstone-conglomerates. In fact, the exposures here 

 included are so closely connected with the greenstone-conglomerates that 

 the lines which separate them are to some extent arbitrary. Where the 

 pebbles disapjjcar from a conglomeratic area, and the rock at the same time 

 has the character of a basic eruptive, it is mapped as a greenstone; but it 

 is probable that the greenstone-conglomerates in the north part of Sec. 14 

 are parts of the same rock mass as the greenstones to the west and east. In 

 macroscopic appearance these eruptives are very like the greenstone- 

 conglomerates. The finer grained of them sometimes take 'on a schistose 

 structure; they have alight green or grayish color, and, in short, so closely 

 resemble the pebbles and some of the matrices of the greenstone-conglom- 

 erates as to be indistinguishable from them. 



The close field association of the eruptives with the greenstone-con- 

 glomerates is still further emphasized by a study of the thin sections. They 

 are all porphyrites, or augite-porphyrites — many of them being amygda- 

 loidal. The liack'ground varies from not very fine grained holocrystalline, 

 to a devitrified glass, the various phases being exact rejDetitious of the peb- 

 bles and matrices of the greenstone-conglomerates. Their macroscopic and 

 microscopic characters are then those of surface flows, and they were doubt- 

 less contemporaneous with the greenstone-conglomerates which have been 

 seen in large part, at least (pp. 377-381), to be volcanic products. 



The exposures in the west part of Sec. 13 have a not very fine grained 

 holocrystalline background. They are augite-porphyrites and, as their 

 structure is a true diabasic one, they would be classed by Rosenbusch as the 

 variety diabase-porphyrite. The menaccanite is often in small rod-like 

 areas, but also has in many cases well defined crystal outlines. As in all the 

 other rocks of the eastern area, it is altered to a greater or less extent to 

 leucoxene. The plagioclase occurs to quite an extent in idiomorphic forms. 

 While much altered, it is fresh enough, so that by Pumpelly's method 



