2 THE GEOLOGY OF MINNESOTA. 



[Fraymental rocks of the Archean. 



or less alteration manifest their derivation from feldspars, from augite and from 

 hornblende, or from an amorphous glassy substance. In the case of much of the 

 greenstone of the Archean this debris is compacted and frequently recrystallized, 

 giving it a striking outward resemblance to a massive rock. But it can sometimes 

 be distinguished from a true igneous massive rock by the occurrence of clastic vari- 

 ations in the texture, and by the peculiar distribution of the crystals which compose 

 it, and also sometimes by the existence of more or less globular, fine, pebble-like 

 masses which become visible especially in the microscopic sections. In these the 

 feldspars are distributed in a radial relation to the surrounding materials the 

 remnant of an original ophitic structure while outside of these pebble-like areas the 

 structure is that of a compact, fine, uniform clastic, though consisting essentially of 

 the same elements. It is only rarely that augite is distinctly preserved amongst 

 this debris, but feldspar is almost always distinctly preserved. The early augites, 

 coming from some ferro-magnesian magma through the action of explosive ejection 

 are apt to be converted to some form of hornblende, frequently actinolite, and this 

 finely disseminated hornblendic mineral is the most potent cause of the prevalent 

 green color of these rocks. The feldspars, which are more or less in fragments, 

 usually have lost their original composition and crystalline purity, and are perme- 

 ated by zoisite and calcite, and less frequently by quartz, epidote and chlorite, and at 

 the same time are clouded by other indefinite, or kaolinic products of decay. On 

 the other hand the feldspars are subject to another form of alteration by which 

 their distinctness, even their outlines, are rendered almost imperceptible. This 

 occurs in some of the later portions of the Archean, and especially in the rocks of 

 the region of Ogishke Muncie lake. The original feldspar is replaced, in whole or in 

 part, by a granular or micro-granulitic complex of quartz, or quartz and glassy feld- 

 spar. In some cases this substitution is so fine-grained that the shape outlined by 

 it appears to be due to a grain of devitrified glass, or of aporhyolyte. It was found 

 that this rearrangement of the feldspars of the Archean becomes coarser textured, 

 and in that form is quite a prevalent mode of regeneration and of metamorphism, 

 the significance of which is important, but which will be treated under the sub- 

 topic of " Metamorphism of the Archean Fragmentals." 



Nearly always any thin section of one of these fragmental volcanic rocks dis- 

 closes more or less of the iron ores. This ore may be pyrite, hematite, magnetite or 

 ilmenite, and very often, in case of original ilmenite, this mineral is altered to 

 leucoxene, and is accompanied by sphene.* In some instances notable amounts of 

 nearly opaque leucoxene have been seen in these greenstones. 



Epidote is quite common, and seems to have been generated in circumstances 

 where alteration was slow and deep-seated, or was aided by other causes than simple 



*Prequently in the vicinity of the iron mines more or less siderite and limonite also occur. 



